CN114830043A - Dark field digital holographic microscope and associated metrology method - Google Patents

Abstract

The present invention discloses a dark field digital holographic microscope configured to determine properties of interest of a structure. The darkfield digital holographic microscope includes an illumination device configured to provide at least a first beam pair comprising a first illumination radiation beam (1010) and a first reference radiation beam (1030 ); and a second beam pair comprising a second illumination radiation beam (1020) and a second reference radiation beam (1040); and one or more optical elements (1070), the one or more optical elements (1070) A plurality of optical elements are operable to capture first scattered radiation scattered by the structure and capture second scattered radiation scattered by the structure, the first scattered radiation and the second scattered radiation, respectively generated by the irradiation beam and the second irradiation beam. The beams of the first beam pair are mutually coherent, and the beams of the second beam pair are mutually coherent. The illumination device is configured to impose incoherence (ADI) between the first beam pair and the second beam pair.

Description

Dark field digital holographic microscopy and associated measurement methods

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European application 19216970.4, filed on December 17, 2019, and European application 20167524.6, filed April 1, 2020, the entire contents of which are incorporated herein by reference.

technical field

The present invention relates to dark field digital holographic microscopy and in particular to high speed dark field digital holographic microscopy, and to metrology applications in the manufacture of integrated circuits.

Background technique

A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. Lithographic apparatuses can be used, for example, in integrated circuit (IC) fabrication. A lithographic apparatus may, for example, project a pattern (also commonly referred to as a "design layout" or "design") at a patterning device (eg, a mask) onto a radiation-sensitive material (anti-radiation) disposed on a substrate (eg, a wafer). etch) layer.

To project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Typical wavelengths currently in use are 365nm (i-line), 248nm, 193nm and 13.5nm. A lithographic apparatus using extreme ultraviolet (EUV) radiation having a wavelength in the range of 4 nm to 20 nm (eg 6.7 nm or 13.5 nm) can be used for Smaller features are formed on the substrate.

Low k ₁ lithography can be used to process features with dimensions smaller than the typical resolution limit of a lithographic apparatus. In such a process, the resolution can be formulated as CD=k ₁ ×λ/NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics in the lithographic apparatus, and CD is the "critical"size" (usually the smallest feature size printed, but in this case half pitch) and k ₁ is the empirical resolution factor. _In general, the smaller ki, the more difficult it is to reproduce patterns on the substrate that resemble shapes and dimensions planned by the circuit designer in order to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and/or design layout. These steps include, for example, but not limited to, optimization of NA, custom illumination schemes, use of phase-shifting patterning devices, various optimizations of design layouts such as optical proximity correction (OPC, sometimes referred to as "optical and process correction"), or other methods commonly defined as "resolution enhancement techniques" (RET). Alternatively, a tight control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of patterns at low _k1 .

During the manufacturing process, the fabricated structures need to be inspected and/or properties of the fabricated structures need to be measured. Suitable inspection and metrology equipment are known in the art. One of the known measurement devices is a scatterometer and eg a dark field scatterometer.

Patent Application Publication US2016/0161864A1, Patent Application Publication US2010/0328655A1 and Patent Application Publication US2006/0066855A1 discuss embodiments of a lithographic apparatus and embodiments of a scatterometer. The cited documents are incorporated herein by reference.

Darkfield microscopes such as the aforementioned metrology devices, and more generally, have problems with angular ranges with limited illumination and detection due to the requirement to share the total angular range (corresponding to the angular range) between the illumination and detection paths. Distinguish the area within the pupil). This situation limits the effective numerical aperture (NA) in illumination and/or detection. The problem of increased effective NA for both illumination and detection has been addressed by implementing an ordered acquisition scheme. Therefore, the measurement speed is undesirably low.

It would be desirable to provide a darkfield microscope with an increased effective NA of at least the detection optics in order to improve resolution by capturing diffracted light over a wider range of diffraction angles.

SUMMARY OF THE INVENTION

In a first aspect of the invention there is provided a darkfield digital holographic microscope configured to determine a property of interest of a structure, the darkfield digital holographic microscope comprising: an illumination device configured to provide at least : a first beam pair comprising a first illuminating radiation beam and a first reference radiation beam; and a second beam pair comprising a second illuminating radiation beam and a second reference radiation beam; and an imaging branch operable to detect at least a first scattered radiation scattered by the structure and a second scattered radiation scattered by the structure, the first scattered radiation being affected by the structure Illuminated by a beam of illumination radiation, the second scattered radiation is generated by illumination of the structure by the second beam of illumination radiation, the imaging branch has a detection NA greater than 0.1; wherein the illumination device is configured such that: the first illuminating radiation beam and the first reference radiation beam are at least partially temporally coherent and at least partially spatially coherent; the second illuminating radiation beam and the second reference radiation beam are at least partially temporal coherent and at least partially spatially coherent; and the illumination device is configured to impose spatial incoherence and/or temporal incoherence between the first beam pair and the second beam pair.

In a second aspect of the invention there is provided a method of determining a characteristic of interest of a target formed on a substrate by a lithographic process, the method comprising: illuminating the target with a first beam of illuminating radiation and capturing a the resulting first scattered radiation scattered from the target; irradiating the target with a second beam of illumination radiation and capturing the resulting second scattered radiation that has been scattered from the target; in including the first beam of illumination and imposing spatial incoherence and/or temporal incoherence between the first beam pair of the reference beam and the second beam pair comprising the second illumination beam and the second reference beam such that: the first beam pair beams of the beam pair are at least partially spatially coherent and at least partially temporally coherent, the beams of the second beam pair are at least partially spatially coherent and at least partially temporally coherent, and the first beam pair any beam is spatially incoherent and/or temporally incoherent with respect to any beam of said second beam pair; and simultaneously generating a second beam resulting from interference of said first scattered radiation with said first reference radiation beam; an interference pattern, and a second interference pattern resulting from interference of the second scattered radiation with the second reference beam.

Description of drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which:

- Figure 1 depicts a schematic overview of a lithographic apparatus;

- Figure 2 depicts a schematic overview of a lithographic cell;

- Figure 3 depicts a schematic representation of monolithic lithography representing the collaboration between three key technologies to optimize semiconductor fabrication;

- Figure 4 depicts a schematic overview of a scatterometry device used as a metrology device, which scatterometry device may comprise a darkfield digital holographic microscope according to an embodiment of the invention;

- Figure 5 depicts a schematic overview of a level sensor device which may comprise a darkfield digital holographic microscope according to an embodiment of the invention;

- Figure 6 depicts a schematic overview of an alignment sensor device which may comprise a darkfield digital holographic microscope according to an embodiment of the invention;

- Figure 7 schematically depicts an example of a diffraction-based darkfield metrology device operating in a parallel acquisition scheme;

- Figure 8 schematically depicts different examples of diffraction-based darkfield metrology devices operating in a continuous acquisition scheme;

- Figure 9 schematically depicts an example of a darkfield digital holographic microscope operating in a continuous acquisition scheme;

- Figure 10 schematically depicts a darkfield digital holographic microscope (df-DHM) operating in a parallel acquisition scheme according to an embodiment;

- Figure 11 schematically depicts an illumination device capable of providing a plurality of radiation beams according to an embodiment;

- Figure 12 schematically depicts an illumination device capable of providing a plurality of radiation beams according to different embodiments;

- Figure 13 depicts a Fourier transformed image in the spatial frequency domain;

- Figure 14 depicts a flowchart of a method for determining the amplitude and phase of a complex field according to another different embodiment; and

- Figure 15 depicts a block diagram of a computer system for controlling a darkfield digital holographic microscope.

Detailed ways

In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (eg, having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and extreme ultraviolet radiation (EUV, eg with wavelengths in the range of about 5 nm to 100 nm).

The terms "reticle," "mask," or "patterning device" as used herein may be interpreted broadly to refer to a general-purpose patterning device that can be used to impart a patterned cross-section to an incoming radiation beam, The patterned cross-section corresponds to the pattern to be produced in the target portion of the substrate. The term "light valve" may also be used in this context. In addition to classical masks (transmissive or reflective, binary, phase shift, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes: an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (eg, UV radiation, DUV radiation, or EUV radiation); a mask support (eg, a mask a stage) MT, the mask support configured to support the patterning device (eg, mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters; A bottom support (eg, wafer stage) WT configured to hold a substrate (eg, a resist-coated wafer) W and connected to a substrate configured to be accurately positioned according to certain parameters a second positioner PW of the substrate support; and a projection system (eg, a refractive projection lens system) PS configured to project the pattern imparted to the radiation beam B by the patterning device MA onto the substrate W on target portion C (eg, including one or more dies).

In operation, illumination system IL receives a radiation beam from radiation source SO, eg, via beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping, ie, shaping, and/or controlling radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and/or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

The term "projection system" PS as used herein should be interpreted broadly to encompass various types of projection systems suitable for the exposure radiation used or for other factors such as the use of immersion liquids or the use of vacuum, Including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid (eg, water) having a relatively high refractive index in order to fill the space between the projection system PS and the substrate W - this is also called for immersion lithography. More information on immersion techniques is given in US6952253, incorporated herein by reference.

The lithographic apparatus LA may also be of the type having two or more substrate supports WT (aka "dual stage"). In such a "multi-platform" machine, the substrate supports WT may be used in parallel, and/or the subsequent preparation of the substrate W may be performed on a substrate W located on one of the substrate supports WT A step of exposing while using another substrate W on another substrate support WT for exposing a pattern on another substrate W.

In addition to the substrate support WT, the lithographic apparatus LA may also include a measurement stage. The measurement platform is arranged to hold the sensor and/or the cleaning device. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement platform can hold multiple sensors. The cleaning device may be arranged to clean parts of the lithographic apparatus, for example parts of the projection system PS or parts of the system providing the immersion liquid. The measurement stage may move under the projection system PS when the substrate support WT is away from the projection system PS.

In operation, the radiation beam B is incident on the patterning device (eg, mask) MA held on the mask support MT, and is patterned by the pattern (design layout) present on the patterning device MA . After traversing the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam on the target portion C of the substrate W. By means of the second positioner PW and the position measurement system IF, the substrate support WT can be moved accurately, eg, in order to position the different target parts C in the path of the radiation beam B at the focus and alignment positions. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in FIG. 1 ) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks P1, P2 as illustrated occupy dedicated target portions, the substrate alignment marks may be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between the target portions C, it is called a scribe line alignment mark.

As shown in FIG. 2 , lithographic apparatus LA may form part of lithographic cells LC (sometimes also referred to as lithographic cells or (lithographic) clusters), which often also include Equipment that performs pre-exposure and post-exposure processes. Conventionally, these include a spin coater SC for depositing the resist layer, a developer DE for developing the exposed resist, eg for adjusting the temperature of the substrate W (eg for adjusting the solvent in the resist layer) The Chilling Plate CH and the Baking Plate BK. The substrate transporter or robot RO picks up the substrates W from the input/output ports I/O1, I/O2, moves the substrates W between the different process equipment and transfers the substrates W to the feed station LB of the lithographic apparatus LA . The devices in the lithography unit, which are also commonly referred to as tracks or coating and developing systems, are usually under the control of a track control unit or coating and developing system control unit TCU, which itself can be controlled by a supervisory control system SCS. The supervisory control system SCS can also control the lithography apparatus LA, eg via the lithography control unit LACU.

In order to properly and consistently expose the substrate W exposed by the lithographic apparatus LA, it is desirable to inspect the substrate to measure properties of the patterned structures, such as overlap error between subsequent layers, line thickness, critical dimension (CD), and the like. For this purpose, inspection tools (not shown) may be included in the lithography cell LC. If errors are detected, the exposure of subsequent substrates or other processing steps to be performed on the substrate W can be adjusted, for example, especially if other substrates W of the same batch or batches are still to be exposed or processed before being checked Down.

Inspection devices, which may also be referred to as metrology devices, are used to determine properties of substrates W, and in particular how properties of different substrates W vary or how properties related to different layers of the same substrate W vary from layer to layer. The inspection apparatus is alternatively configured to identify defects on the substrate W, and may eg be part of the lithographic cell LC, or may be integrated into the lithographic apparatus LA, or may even be a separate device. The inspection equipment can measure one or more properties with respect to the latent image (image in the resist layer after exposure), or with respect to the semi-latent image (image in the resist layer after the post-exposure bake step PEB). , or with respect to one or more properties of the developed resist image (where exposed or unexposed portions of the resist have been removed), or even with respect to the etched image (in patterns such as etched pattern transfer) step) one or more properties.

In general, the patterning process in the lithographic apparatus LA is one of the most important steps in the process, which requires a high degree of accuracy in the dimensioning and placement of the structures on the substrate W. To ensure this high accuracy, the three systems can be combined in a so-called "holistic" control environment schematically depicted in FIG. 3 . One of these systems is the lithographic apparatus LA, which is (virtually) connected to the metrology tool MT (second system) and connected to the computer system CL (third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and provide a tight control loop to ensure that the patterning performed by the lithographic apparatus LA remains within the process window. A process window defines the range of process parameters (eg, dose, focal length, overlap) within which a particular manufacturing process produces a defined result (eg, functional semiconductor device)—typically within the defined range, Process parameters are allowed to vary during the lithography process or patterning process.

The computer system CL can use the design layout(s) to be patterned to predict which resolution enhancement technique to use, as well as perform computational lithography simulations and calculations to determine which mask layout and lithographic apparatus settings enable the patterning process. Maximum overall process window (depicted in Figure 3 by the double arrow in the first scale SC1). Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL can also be used to detect where within the process window the lithographic apparatus LA is currently operating (eg, using input from the metrology tool MT) to predict whether there may be defects due to, for example, suboptimal processing (by a second scale). The arrow pointing to "0" in SC2 is depicted in Figure 3).

The metrology tool MT can provide input to the computer system CL to enable accurate simulations and predictions, and can provide feedback to the lithographic apparatus LA to identify possible drifts, such as in the calibration state of the lithographic apparatus LA (represented in FIG. Multiple arrows in the third scale SC3 depict).

During lithography, frequent measurements of the resulting structures are desirable, eg for process control and verification. The tool used to make such a measurement is often referred to as a measurement tool MT. Different types of metrology tools MT are known for making these measurements, including scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer is a multifunctional instrument that allows the measurement of parameters of a lithographic process by having a sensor in the pupil or in a plane that is conjugate to the pupil of the scatterometer's objective (measurement is often referred to as pupil-based measurement), or Parameters of the lithographic process are measured by having sensors in the image plane or in a plane conjugate to the image plane, in which case the measurement is often referred to as image or field-based measurement. These scatterometers and related measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers can measure gratings using light from soft x-rays and visible to near-IR wavelength ranges.

In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such scatterometers, reconstruction methods can be applied to the measured signal to reconstruct or calculate properties of the grating. Such reconstruction may be caused, for example, by simulating the interaction of scattered radiation with a mathematical model of the target structure and comparing the simulated results with those measured. Adjust the parameters of the mathematical model until the simulated interaction produces a diffraction pattern similar to that observed from a real target.

In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target and reflected or scattered radiation from the target is directed onto a spectrometer detector that measures the spectrum of the specularly reflected radiation (ie, A measure of intensity as a function of wavelength). From such data, the structure or profile of the target producing the detected spectrum can be reconstructed, for example, by rigorous coupled wave analysis and nonlinear regression, or by comparison with a library of simulated spectra.

In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. Ellipsometry scatterometers allow parameters of the lithographic process to be determined by measuring scattered radiation for each polarization state. Such a measuring device emits polarized light (such as linearly polarized light, circularly polarized light or elliptically polarized light) by using, for example, a suitable polarizing filter in the illumination section of the measuring device. A source suitable for the measurement device may also provide polarized radiation. Various embodiments of existing ellipsometry scatterometers are described in US patent applications 11/451,599, 11/708,678, 12/256,780, 12/486,449, 12/920,968, 12/922,587, 13/000,229, 13/033,135, 13/533,110 and 13/891,410.

Measurement equipment, such as a scatterometer, is depicted in FIG. 4 . The metrology equipment comprises a broadband (white light) radiation projector 2 that projects radiation onto a substrate W. The reflected or scattered radiation is passed to a spectrometer detector 4 which measures the spectrum 6 of the specularly reflected radiation (ie a measure of intensity as a function of wavelength). From this data, the detected spectrum can be reconstructed by the processing unit PU (eg, by rigorous coupled wave analysis and nonlinear regression, or by comparison with a library of simulated spectra as shown at the bottom of Figure 3) structure or outline 8. Typically, for reconstruction, the general form of the structure is known, and some parameters are assumed from knowledge of the process used to manufacture the structure, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer may be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer.

The overall measurement quality of a lithographic parameter obtained via measurement of a metrology target is determined, at least in part, by the measurement option used to measure the lithographic parameter. The term "substrate measurement option" may include measuring one or more parameters of itself, one or more parameters of one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement option is a diffraction-based optical measurement, one or more of the measured parameters may include the wavelength of the radiation, the polarization of the radiation, the incidence of the radiation with respect to the substrate angle, orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria used to select the measurement option may be, for example, the sensitivity of one of the measurement parameters to process variations. Further examples are described in US patent application US2016-0161863 and published US patent application US 2016/0370717A1, which are incorporated herein by reference in their entirety.

Another type of metrology tool used in IC manufacturing is a topography measurement system, level sensor or height sensor. Such a tool can be integrated into the lithographic apparatus for measuring the top surface of a substrate (or wafer). A topography map (also referred to as a height map) of the substrate can be generated from these measurements indicative of the height of the substrate as a function of position on the substrate. This height map can then be used to correct the position of the substrate during transfer of the pattern onto the substrate to provide space for the patterning device in the proper focus position on the substrate image. It will be understood that "height" in this context refers to a dimension (also referred to as the Z-axis) that is substantially out-of-plane relative to the substrate. Typically, a level or height sensor performs measurements at a fixed location (relative to its own optical system), and relative motion between the substrate and the optical system of the level or height sensor causes multiple Height measurement at the site.

An example of a level or height sensor LS as known in the art is shown schematically in Fig. 5, which diagram only illustrates the principle of operation. In this example, the level sensor includes an optical system including a projection unit LSP and a detection unit LSD. Said projection unit LSP comprises a radiation source LSO providing a radiation beam LSB imparted by the projection grating PGR of the projection unit LSP. The radiation source LSO may be, for example, a narrowband or broadband light source (such as a supercontinuum light source), a polarized or unpolarized, pulsed or continuous light source (such as a polarized or unpolarized laser beam). The radiation source LSO may comprise a plurality of radiation sources having different colors, or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not limited to visible light radiation, but may additionally or alternatively cover UV and/or IR radiation and any wavelength range suitable for reflection from the surface of the substrate.

The projection grating PGR is a periodic grating comprising a periodic structure which produces a radiation beam BE1 with periodically varying intensity. A radiation beam BE1 having a periodically varying intensity is directed towards the measurement site MLO on the substrate W, said radiation beam BE1 having a relative angle between 0 and 90 degrees, typically between 70 and 80 degrees. Incidence angle ANG on the axis perpendicular to the surface of the incident substrate portion (Z axis). At the measurement site MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2 ) and directed towards the detection unit LSD.

To determine the height level at the measurement site MLO, the level sensor further includes a detection system including a detection grating DGR, a detector DET, and a processing unit (not shown) for processing an output signal of the detector DET. Shows). The detection grating DGR may be the same as the projection grating PGR. The detector DET produces a detector output signal indicative of the received light, eg indicative of the intensity of the received light (such as a light detector), or indicative of a spatial distribution of the received intensity (such as a camera ). The detector DET may comprise any combination of one or more detector types.

By means of triangulation techniques, the height level at the measurement site MLO can be determined. The detected height level is generally related to the signal strength as measured by the detector DET, which signal strength has a periodicity that depends inter alia on the (oblique) angle of incidence ANG and the design of the projection grating PGR.

The projection unit LSP and/or the detection unit LSD may also comprise other optical elements along the path (not shown) of the patterned radiation beam between the projection grating PGR and the detection grating DGR , such as lenses and/or mirrors.

In an embodiment, the detection grating DGR may be omitted, and the detector DET may be placed where the detection grating DGR is located. This configuration provides a relatively direct detection of the image of the projected grating PGR.

In order to effectively cover the surface of the substrate W, the level sensor LS can be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby creating a measurement area MLO or spot covering a larger measurement range array of .

Various height sensors of general type are disclosed, for example, in US7265364 and US7646471, which are incorporated by reference. Altitude sensors using UV radiation instead of visible or infrared radiation are disclosed in US2010233600A1, which is incorporated by reference. In WO2016102127A1, which is incorporated by reference, a compact height sensor is described that uses a multi-element detector to detect and identify the position of a grating image without the need to detect gratings.

Another type of metrology tool used in IC manufacturing is alignment sensors. Therefore, a key aspect of the performance of the lithographic apparatus is the ability to properly and accurately place the applied pattern relative to features (by the same apparatus or a different lithographic apparatus) laid out in previous layers. For this purpose, the substrate is provided with one or more sets of marks or targets. Each marker is a structure whose position can be measured later using a position sensor, typically an optical position sensor. The position sensors may be referred to as "alignment sensors" and the marks may be referred to as "alignment marks".

The lithographic apparatus may include one or more (eg, multiple) alignment sensors by which the positions of alignment marks disposed on the substrate may be accurately measured. Alignment (or position) sensors may use optical phenomena such as diffraction and interference to obtain positional information from alignment marks formed on the substrate. An example of an alignment sensor used in current lithographic apparatuses is based on a self-referencing interferometer as described in US6961116. Various enhancements and modifications of the position sensor have been developed, eg as disclosed in US2015261097A1. All of these publications are incorporated herein by reference.

Figure 6 is a schematic block diagram of an embodiment of a known alignment sensor AS such as described, for example, in US6961116 and incorporated by reference. The radiation source RSO provides a radiation beam RB having one or more wavelengths, which is turned by the steering optics onto a mark, such as the mark AM on the substrate W, as an illumination spot SP. In this example, the turning optics include a spot mirror SM and an objective OL. The diameter of the irradiation spot SP irradiating the mark AM may be slightly smaller than the width of the mark itself.

The radiation diffracted by the alignment mark AM (via the objective OL in this example) is collimated into an information-bearing beam IB. The term "diffraction" is intended to include zero order diffraction (which may be referred to as reflection) from the marking. A self-referencing interferometer SRI of the type disclosed for example in the above-mentioned US6961116 interferes with itself with the beam IB, which is then received by the photodetector PD. Additional optics (not shown) may be included to provide multiple individual beams if more than one wavelength is generated by the radiation source RSO. The light detector may be a single element, or it may comprise a number of pixels as desired. The light detector may comprise a sensor array.

The turning optics, which in this example comprise the spot mirror SM, can also be used to block zero-order radiation reflected from the markings, so that the information-bearing beam IB only comprises higher-order radiation from the markings AM order diffracted radiation (this is not necessary for the measurement, but improves the signal-to-noise ratio).

The strength signal SI is supplied to the processing unit PU. By combining the optical processing in the block SRI with the computational processing in the unit PU, the values of the X position and the Y position on the substrate relative to the reference frame are output.

A single measurement of the illustrated type only fixes the position of the markers within a certain range corresponding to a spacing of the markers. Coarse measurement techniques are used in conjunction with this measurement to identify which cycle of the sine wave is the cycle containing the marked location. The same process at a coarser and/or finer level can be repeated at different wavelengths for improved accuracy and/or for robust, ie robust detection of the marker, while making The material of the indicia is independent of the material on which the indicia is placed above and/or below. The wavelengths may be optically multiplexed and demultiplexed to process the wavelengths simultaneously, and/or the wavelengths may be multiplexed by time or frequency division.

In this example, the alignment sensor and spot SP remain stationary while the substrate W moves. Thus, the alignment sensor can be rigidly and accurately mounted to the reference frame, while at the same time efficiently scanning the marks AM in the direction opposite to the direction of movement of the substrate W. In such movement, the substrate W is controlled by mounting the substrate W on a substrate support and controlling the movement of the substrate support by a substrate positioning system. A substrate support position sensor (eg, an interferometer) measures the position of the substrate support (not shown). In an embodiment, one or more (alignment) marks are provided on the substrate support. Measurement of the position of the marks provided on the substrate support allows calibration of the position of the substrate support as determined by the position sensor (eg, connected relative to the alignment system) to the frame). Measurement of the position of the alignment marks provided on the substrate allows to determine the position of the substrate relative to the substrate support.

To monitor the lithography process, parameters of the patterned substrate are measured. Parameters may include, for example, overlap errors between successive layers formed in or on the patterned substrate. Such measurements can be performed on product substrates and/or on dedicated metrology targets. Various techniques exist for making measurements of the microstructures formed in the lithographic process, including the use of scanning electron microscopes and various specialized tools. A specialized inspection tool in a rapid and non-invasive form is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate, and the properties of the scattered or reflected beam are measured.

Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. The targets used by such scatterometers are relatively large (eg, 40 μm by 40 μm) gratings, and the measurement beam produces a smaller spot than the grating (ie, the grating is underfilled). In addition to the measurement of feature shapes by reconstruction, diffraction-based overlap can also be measured using this device, as described in published patent application US2006066855A1. Diffraction-based overlay measurements using darkfield imaging of diffractive orders enable overlay measurements on smaller targets. Examples of darkfield imaging measurements can be found in international patent applications WO 2009/078708 and WO 2009/106279, the entire contents of which are hereby incorporated by reference. Further developments of the described technology have been described in published patent publications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A and WO2013178422A1. These targets can be smaller than the illumination spot and can be surrounded by product structures on the wafer. Multiple rasters can be measured in one image using a composite raster objective. The contents of all of these applications are also incorporated herein by reference.

Darkfield microscopes such as the aforementioned metrology devices, and more generally, have problems with a limited angular range for detection of light diffracted by and/or illumination of the target, as it may be required that the illumination path differs from the detection. The total angular extent (corresponding to the region within the angle-resolving pupil) is shared between the paths. This situation limits the effective NA in illumination and detection.

In a diffraction-based darkfield metrology device, a beam of radiation is directed onto a metrology target, and one or more properties of the scattered radiation are measured in order to determine properties of interest for the target. The properties of the scattered radiation may include, for example, the intensity at a single scattering angle (eg, as a function of wavelength), or the intensity at one or more wavelengths as a function of scattering angle.

The measurement of the target in the dark field measurement may include, for example, measuring the first intensity I ₊₁ of the 1st diffraction order and the second intensity (I _-1 ) of the -1st diffraction order, and calculating the intensity asymmetry (A=I ₊₁ -I _-1 ), the intensity asymmetry is indicative of the asymmetry on the target. The metrology target may include one or more grating structures from which parameters of interest can be inferred from such intensity asymmetry measurements, eg, the target is designed for so that the asymmetry in the target varies with the parameter of interest. For example, in overlay metrology, the target may comprise at least one composite grating formed from at least one pair of overlapping sub-gratings that are patterned in different layers of the semiconductor device. The asymmetry of the target will therefore depend on the alignment of the two layers and thus on the overlap. Other targets may be formed with structures that are exposed to varying degrees based on the focus settings used during exposure; measurements made on them enable the focus settings to be inferred inversely (again via intensity asymmetry).

7 and 8 schematically illustrate two examples of diffraction-based dark field metrology devices. Note that, for simplicity, the two figures only show some of the components that are sufficient for the purpose of describing the working principle of the two devices.

As illustrated in FIG. 7 , the first illuminating radiation beam IB1 may be incident obliquely on the overlapping target of the substrate WA from one side of the device. The grating-based overlapping target can diffract the first illumination beam into a certain number of diffraction orders. Because the device is configured for darkfield imaging, the zero diffraction order may be blocked by optics, or configured to fall completely outside the numerical aperture of the objective OB. At least one non-zero diffraction order, eg a positive first diffraction order (+1 ^st DF) can be collected by the objective OB. At the pupil plane of the objective OB, the first wedge WG1 can be used to redirect the diffracted radiation to follow the desired beam path. Finally, the imaging lens can be used to focus the diffractive order, eg the positive first diffractive order (+1 ^st DF), on the image sensor IS, so that the first image IM1 is formed at the first location.

Similarly, a second beam of illuminating radiation IB2 may be incident obliquely on the same overlapping target OT of the substrate WA from the opposite side of the system. The incident angle of the second irradiation beam IB2 may be the same as the incident angle of the first irradiation beam IB1. At least one non-zero diffraction order, eg the negative first diffraction order (-1 ^st DF) can be collected by the objective OB and subsequently redirected by the second wedge WG2. The negative first diffraction order (-1 ^st DF) can then be focused on the image sensor IS by the imaging lens IL, so that a second image IM2 is formed at the second location.

The example of Figure 7 is operated in a parallel acquisition scheme. The overlapping target is simultaneously irradiated by both irradiation beams IB1, IB2. Correspondingly, the two spatially separated images IM1, IM2 of the overlapping target are acquired simultaneously. Such a parallel acquisition scheme allows fast measurement speeds and thus high throughput. However, the pupil plane of the objective lens OB must be shared by two diffraction orders, eg the positive first diffraction order (+1 ^st DF) and the negative first diffraction order (-1 ^st DF). As a result of dividing the pupil into mutually exclusive illumination pupils and detection pupils, there is a consequent reduction in illumination NA as well as in detection NA. While there is some flexibility in the tradeoff between illumination NA and detection NA, it is ultimately not possible to have both illumination NA, detection NA as large as is usually desired within a single pupil. This situation results in a limited angular range for each corresponding illumination beam and for the positive first diffracted order (+1 ^st DF) and the negative first diffracted order (−1 ^st DF) beams, such Circumstances in turn limit the allowable grating pitch size and/or range of illumination wavelengths, and thus impose stringent requirements for designing such metrology systems.

Figure 8 schematically illustrates another exemplary dark field metrology device (or a different mode of operation of the device of Figure 7). The main difference is that the measurement device of Figure 8 operates in a continuous acquisition scheme. In a continuous acquisition scheme, the measurement target OT is illuminated from one direction at any time by only one illumination beam, and thus only one image of said target is formed and acquired at any point in time. Referring to Figure 8, at a first time instance t=T1, the first illumination beam IB1 may be switched on and directed obliquely from one side of the metrology device onto the overlapping target OT of the substrate WA. After interaction with the grating of the overlapping target, multiple diffraction orders can be created. At least one of the non-zero diffraction orders, eg the positive first diffraction order (+1 ^st DF), can be collected by the objective OB and subsequently focused on the image sensor IS by the imaging lens IL.

After the first image IM1 of the overlapping grating has been acquired, at a second time instance t=T2, the first illumination beam IB1 is turned off and the second illumination beam IB2 is turned on. The second illuminating radiation beam IB2 may be directly incident obliquely on the same overlapping target from the opposite side of the measurement device. At least one of the resulting diffraction orders, eg the negative first diffraction order (-1 ^st DF), can be collected by the objective OB and subsequently focused on the image sensor IS to form the overlapping target of the second image IM2. It should be noted that both images IM1 and IM2 may be formed at a common location on the image sensor.

With this time-multiplexed acquisition scheme, the full NA of objective OB is made available to detect the diffracted beams, ie the positive first diffraction order (+1 ^st DF) and the negative first diffraction order (-1 ^st DF) DF). Unrestricted objective NA means that a wider range of design parameters such as grating pitch size, illumination wavelength and illumination angle is allowed and greater flexibility in system design can be obtained. However, the fact that multiple image acquisitions are required means that the measurement speed is reduced and thus affects the system throughput.

Furthermore, an accurate determination of, for example, the overlap error relies on an accurate measurement of the smallest relative intensity difference (or intensity asymmetry) between the two acquired images IM1, IM2. A typical relative intensity difference is on the order of about ^10-4 of the intensity of one of the acquired images (eg, IM1 or IM2). Such small intensity differences can be easily outweighed by any fluctuations in intensity and/or wavelength of the illuminating radiation. Therefore, the illumination beam is required to remain stable during successive image acquisitions. This situation can be achieved by using a stable light source that provides the desired intensity and wavelength stability. Alternatively, additional hardware and software such as eg intensity/wavelength monitoring means and corresponding feedback control loops should be incorporated into the measurement means so that the intensity and/or wavelength fluctuations of the illumination beam are actively monitored and well controlled. compensate. In some cases, an intensity monitoring device may be used to actively track the intensity of the illumination beam. The signal generated from the intensity monitoring device can be used (eg, electronically) to correct for fluctuations in the intensity of the illumination beam. All of these schemes add complexity and cost to the overall system.

Some or all of the aforementioned problems can be solved by using digital holographic microscopy, particularly dark field digital holographic microscopy. Digital holographic microscopy is an imaging technique that combines holography and microscopy. Unlike other microscopy methods that record projected images of objects, digital holographic microscopy records holograms formed by interference between object radiation obtained by illuminating a three-dimensional (3D) object and reference radiation, said The reference radiation is coherent with the object radiation. Images can be captured using, for example, a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS). Since the object radiation is radiation scattered from the object, the wavefront of the object radiation is modulated or shaped by the object. The scattered radiation may comprise reflected radiation, diffracted radiation or transmitted radiation. Thus, the wavefront radiated by an object carries information about the irradiated object, such as 3D shape information. Based on the captured image of the hologram, the image of the object can be numerically reconstructed by using a computer reconstruction algorithm. An important advantage of hologram-based measurements over intensity-based measurements, as described in Figures 7 and 8, is that hologram-based measurements allow to obtain both intensity and phase information of an object. With the additional phase information, the properties of the object can be determined with better accuracy.

International patent application WO2019197117A1, incorporated herein by reference, discloses a method and metrology apparatus for determining properties (eg, overlap) of structures fabricated on a substrate based on dark-field digital holographic microscopy (df-DHM). Figure 3 of International Patent Application WO2019197117A1 is reproduced in Figure 9 for the purpose of description. Figure 9 schematically illustrates the disclosed df-DHM specially adapted for use in lithographic process metrology.

Compared to the previous examples shown in Figures 7 and 8, the df-DHM in Figure 9 also comprises reference optical units 16, 18 for providing two further reference radiation beams 51, 52 (the reference radiation). Such two reference radiation beams 51, 52 are respectively paired with two corresponding parts 41, 42 of the scattered radiation beams 31, 32 (the object radiation). Two scattered reference beam pairs are used to form two interference patterns in succession. Coherence control is provided by adjusting the relative optical path-length difference (OPD) between the two scattered reference beams within each beam pair. However, coherence control is not available between two beam pairs.

Due to the use of a single light source and insufficient coherence control, then all four radiation beams, namely the first portion 41 of the scattered radiation 31, the first reference radiation 51, the second portion 42 of the scattered radiation 32, and the second reference radiation 52 are mutually relevant. If these four mutually coherent radiation beams are allowed to arrive at the same location of the sensor 6 simultaneously, ie operating in a parallel acquisition scheme, multiple The patterns will overlap each other. Undesired interference patterns may be formed by, for example, interference between portions 41 of the first scattered radiation 31 and portions 42 of the second scattered radiation 32 . Parallel acquisition is impractical for this arrangement as it is technically challenging and time consuming to completely separate the superimposed interference patterns.

Similar to the example of Figure 8, using a continuous acquisition scheme in the example of Figure 9 allows the full NA of the objective to be used for both illumination and detection. However, the system suffers from the same problem of low measurement speed due to continuous acquisition. Therefore, it is desirable to have a df-DHM that can perform parallel acquisitions so that both high measurement speed and high design flexibility can be obtained.

Figure 10 schematically illustrates the imaging branch of a dark field digital holographic microscope (df-DHM) 1000 according to an embodiment. Dark field digital holographic microscopy (df-DHM) includes an imaging branch and an illumination branch. In such an embodiment, the metrology target 1060 comprising the structures located on the substrate 1050 is illuminated by two illumination radiation beams, namely, a first illumination radiation beam 1010 and a second illumination radiation beam 1020 . In an embodiment, such two illumination beams 1010, 1020 may illuminate the metrology target 1060 simultaneously.

In an embodiment, the first illumination beam 1010 may be incident on the measurement target 1060 at a first incident angle in a first direction with respect to the optical axis OA. The second illumination beam 1020 may be incident on the measurement target 1060 at a second incident angle in a second direction with respect to the optical axis OA. The first angle of incidence of the first illumination beam 1010 and the second angle of incidence of the second illumination beam 1020 may be approximately the same. The angle of incidence of each illumination beam may, for example, be in the range of 70 to 90 degrees, in the range of 50 and 90 degrees, in the range of 30 to 90 degrees, in the range of 10 to 90 degrees. Illumination of the measurement target 1060 may cause radiation to scatter from the target. In an embodiment, the first illumination beam 1010 may be incident on the measurement target 1060 at a first azimuth angle corresponding to the first direction. The second illumination beam 1020 may be incident on the measurement target 1060 at a second azimuth angle corresponding to the second direction. The first azimuth angle of the first illumination beam 1010 and the second azimuth angle of the second illumination beam 1020 may be different; for example, opposite angles that are spaced apart by 180 degrees are opposite angles.

Depending on the structure of the measurement target 1060, the scattered radiation may comprise reflected radiation, diffracted radiation or transmitted radiation. In such an embodiment, the metrology target may be a diffraction-based overlapping target; and each illumination beam may correspond to a scattered beam comprising at least one non-zero diffraction order. Each scattered beam carries information about the illuminated measurement target. For example, the first illumination beam 1010 may correspond to the first scattered beam 1011 comprising the positive first diffraction order (+1 ^st DF); the second illumination beam 1020 may correspond to the negative first diffraction order (+1 st DF). -1 ^st DF) of the second scattered beam 1021. The zero diffraction order and other undesired diffraction orders can either be blocked by a beam blocking element (not shown), or be configured to fall completely outside the NA of the objective lens 1070 . Therefore, the df-DHM can be operated in dark field mode. It should be noted that in some embodiments, one or more optical elements (eg, lens combinations) may be used to achieve the same optical effect of objective lens 1070 .

Due to the smaller size of the measurement target 1060, the imaging branch may have a net positive magnification (eg, greater than 10× (ie, greater than 10×), greater than 20× (ie, greater than 20×), or greater than or equal to 30× ( i.e. greater than or equal to 30 times)).

Both scattered beams 1011 , 1021 may be collected by objective lens 1070 and then refocused onto image sensor 1080 . It should be noted that the objective 1070 of the imaging branch may be an imaging objective used only in the detection path (as shown) and not used for illumination (eg, to focus illumination on the measurement target 1060). As such, the illumination does not necessarily have to pass through the same objective as the scattered light passes through. In other embodiments, an objective lens may be shared between the imaging branch and the illumination branch to collect the scattered radiation and focus the illumination on the measurement target 1060 .

It is desirable to collect as much scattered/diffracted light as possible, and so high NA detection paths or imaging branches are desirable. For example, in this regard, the high NA may be greater than 0.1, greater than 0.2, greater than 0.3, or greater than 0.4. In other embodiments, a high NA may refer to an NA of 0.8 or greater.

Objective 1070 may include multiple lenses, and/or df-DHM 1000 may include a lens system having two or more lenses (eg, an imaging lens and an objective similar to the exemplary df-DHG of FIG. 9 ), thereby defining The pupil plane of the objective between the two lenses and the image plane at the focal point of the imaging lens. In such an embodiment, a portion 1012 of the first scattered beam 1011 and a portion 1022 of the second scattered beam 1021 are simultaneously incident on the image sensor 1080 at a common location. At the same time, two reference radiation beams, namely the first reference beam 1030 and the second reference beam 1040, are incident on the same location of the image sensor 1080. Such four beams can be grouped into two pairs of scattered radiation and reference radiation. For example, the first scattered reference beam pair may include the portion 1012 of the first scattered beam 1011 and the first reference beam 1030 . Likewise, the portion 1022 of the second scattered reference beam pair may include a second scattered beam 1021 and a second reference beam 1040 . The two scattered reference beam pairs may then form two interference patterns (holographic images) that overlap at least partially in the spatial domain.

In an embodiment, to separate two at least partially spatially overlapping interference patterns (eg, in the spatial frequency domain), the first reference beam 1030 may have a first angle of incidence relative to the optical axis OA, and the The second reference beam 1040 may have a second angle of incidence relative to the optical axis OA; the first angle of incidence and the second incidence are different. Alternatively or additionally, the first reference beam 1030 may have a first azimuth angle relative to the optical axis OA, and the second reference beam 1040 may have a second azimuth angle relative to the optical axis OA; the first azimuth angle and the second azimuth angle Angles are different.

To generate an interference pattern, the two beams of each scattered reference beam pair should be at least partially coherent with each other to a degree sufficient to form an interference pattern. It should be noted that each scattered radiation beam may have a phase offset relative to its corresponding illuminating radiation. For example, at the image plane of image sensor 1080, such a phase shift may include due to the optical path length or optical path (OPD) from measurement target 1060 to image sensor 1080 and by interaction with the measurement target contribution made. As described above, it is necessary to control the coherence between the first pair of scattered reference beams and the second pair of scattered reference beams such that each beam of a pair is incoherent with any beam of the other pair . In other words, interference should only occur between beams within the same beam pair and be suppressed between different beam pairs. In this way, only the desired interference patterns (eg, two interference patterns formed by corresponding pairs of scattered reference beams) are formed on the image sensor 1080 in a superimposed manner, thus avoiding separation or removal of the undesired interference pattern problem.

More particularly, the coherence between beams within the same beam pair should be both temporal and spatial. The mutual coherence function between these beams depends on space and time. A typical approximation or approximation is to factorize such a function into spatial and temporal components. However, when the beam is traveling at some angle (eg, in an off-axis system), this approximation is no longer ideal. There should be sufficient coherence such that the object beam and the reference beam interfere on the camera. For the simplification of temporal and spatial coherence, this situation means that on the camera, each point has the same optical path distance from the source, ie the same optical path length (eg, within the "temporal coherence" length). For limited spatial coherence, each point of the reference beam should also be mapped to the corresponding point of the object beam (this situation may mean that the object and reference arm should get an image of the beam splitter on the camera). In the case of single mode fibers, the spatial coherence can be very large.

In addition to being spatially and temporally coherent with the reference beam (of the same beam pair), each object beam may be smooth (eg, uniformly illuminated) over the full area of the overlapping target.

In an embodiment, the two illumination beams 1010, 1020 and the two reference beams 1030, 1040 used in the df-DHM of Figure 10 may be provided by an illumination branch comprising an illumination device. FIG. 11 schematically illustrates an irradiation apparatus according to an embodiment. As shown in FIG. 11 , the light source 1110 may emit an at least partially coherent primary radiation beam 1111 . The main radiation beam 1111 may include wavelengths ranging from soft x-rays and visible to near IR. The main radiation beam 1111 may be split by the first beam splitter 1120 into two beams, a first radiation beam 1112 and a second radiation beam 1114 . In such an embodiment, the first beam splitter 1120 may include a 50/50 split ratio, and thus the first radiation beam 1112 and the second radiation beam 1114 may have approximately the same power level. Subsequently, the two beams 1112, 1114 may each follow two different beam paths.

In one of the beam paths 1112, 1114 (the second beam path in the example shown here, although this is largely arbitrary), the second radiation beam 1114 (or the first beam 1112) may suffer delays. In the example shown here, the delay is implemented via an incoherent delay arrangement such as an tunable optical delay line AD1 including a prism 1132 . The tunable optical delay line AD1, or more generally the delay, may be used to control the OPD (or coherence) between the beams in the first beam path and the beams in the second beam path. This can be done to ensure that the beams are not coherent so that the first beam pair does not interfere with the second beam pair. Instead of such a delay line, a "hard" path difference may be intentionally introduced between the beam paths 1112, 1114.

In embodiments, the tunable optical delay line AD1 may be operable such that the time delay between the two paths may be maintained as short as possible in order to have similar intensity fluctuations in the two beams, while still imposing non- coherence.

In the first beam path, the first radiation beam 1112 may enter a second beam splitter 1122, which may split the first radiation beam 1112 into two other beams, the first illumination beam 1010 and The first reference beam 1030. Depending on the splitting ratio of the second beam splitter 1122, the first illumination beam 1010 and the first reference beam 1030 may have different powers. The split ratio of the second beam splitter 1122 may be 90/10, 80/20, 70/30, 60/40 or 50/50. In such an embodiment, the power of the first illumination beam 1010 may be higher than the power of the first reference beam 1030 . Each of the two beams 1010, 1030 may then be reflected by a reflective element 1140 into an optical delay line. Each optical delay line, fixed or tunable, may include reflective optical elements to retroactively reflect, ie retro-reflect, incident radiation. The reflective optical element may be a right angle prism 1130 or 1131. In some embodiments, the reflective optical element may be a pair of mirrors. In such an embodiment, the first illumination beam 1010 may pass through a fixed optical delay line including prism 1131 , while the first reference beam 1030 may pass through an adjustable optical delay line AD2 including prism 1130 . In various embodiments, the first illumination beam 1010 may pass through a tunable optical delay line including prism 1130 , while the first reference beam 1030 may pass through a fixed optical delay line including prism 1131 . In either of these two scenarios, the relative OPD between the two beams 1010, 1030 may be adjustable. The two beams 1010, 1030 form a first pair of output beams. It should be noted that this is only one example of an adjustable path length arrangement for enabling adjustment of the OPD between beams 1010, 1030. Any other suitable arrangement for achieving this could be used instead.

The second radiation beam 1114 may be split by the third beam splitter 1124 into two beams, eg, a second illumination beam 1020 and a second reference beam 1040 . Depending on the splitting ratio of the third beam splitter 1124, the two beams may have different powers. In such an embodiment, the split ratio of the third beam splitter 1124 may be the same as the split ratio of the second beam splitter 1122, such that the first illumination beam 1010 and the second illumination beam 1020 may have approximately the same power level , and the first reference beam 1020 and the second reference beam 1040 may have approximately the same power level. In such an embodiment, the power of the second illumination beam 1020 may be higher than the power of the second reference beam 1040 . The two beams 1020, 1040 may then be reflected by the reflective element 1142 into two optical delay lines, one fixed and the other tunable, respectively. In such an embodiment, the second illumination beam 1020 may pass through a fixed optical delay line including prism 1133 , while the second reference beam 1040 may pass through optical delay line AD3 including prism 1134 . In various embodiments, the second illumination beam 1020 may pass through a tunable optical delay line including 1134, while the second reference beam 1040 may pass through a fixed optical delay line including 1133. In either of these two scenarios, the relative OPD between the two bundles 1020, 1040 may be adjustable. The two beams 1020, 1040 form a second pair of output beams. It should be noted that this is only one example of an adjustable path length arrangement that may be used to enable adjustment of the optical path length between beams 1020, 1040. Any other suitable arrangement for achieving this could be used instead.

After leaving/exiting their respective optical delay lines, the four radiation beams, namely the first illumination beam 1010, the first reference beam 1030, the second illumination beam 1020 and the second reference beam 1040, may exit/exit the illumination device 1100, And can be used as illumination beam and reference beam in df-DHM (eg in the corresponding beam of df-DHM of Fig. 10). In embodiments, some or all of these four beams may pass through additional optical elements such as beam shaping elements, optical steering mirrors, optical polarizing elements, and optical power, respectively, before exiting/exiting the illumination device 1100 control element) so that the beam parameters, propagation direction, polarization state and/or optical power of each beam can be independently controlled. Beam parameters may include beam shape, beam diameter, and beam divergence. In an embodiment, the two beams of either of the first pair of output beams and the second pair of output beams may have different power levels. One beam of the first pair of output beams may have approximately the same power level as one beam of the second pair of output beams.

The direction of propagation of the radiation beam exiting/leaving from the illuminator of FIG. 11 determines the angle of incidence and azimuth of the beam relative to the optical axis OA of the df-DHM of FIG. 10 . The orientation of the Cartesian reference frame is shown on the top of FIG. 10 . The angle of incidence of a beam refers to the angle in the x-z plane between the optical axis (dashed line) or z axis of the microscope and the incident beam or its projection in the x-z plane. The azimuth of the beam refers to the angle between the x-axis and the incident beam or its projection in the x-y plane.

Correspondingly, the optical delay line AD2 can be used to adjust the relative OPD between the first illumination beam 1010 and the first reference beam 1030, while the third optical delay line AD3 can be used to adjust the second illumination beam 1020 and the second reference beam at the same time Relative OPD between 1040. Each scattered beam radiation (eg, the first scattered beam 1011 or the second scattered beam 1011 or the second scattered radiation beam, for example, can be independently controlled or optimized as long as the relative OPD induced phase delay is sufficient to cover the phase shift between each illuminating radiation beam and its associated scattered radiation beam). Coherence between scattered beam 1021) and its counterpart reference beam radiation (eg, first reference beam 1030 or second reference beam 1040). Additionally, the tunable optical delay line AD1 can be used to intentionally add two pairs of illumination reference beams or two pairs of scattered reference beams (eg, a first scattered reference including portion 1012 of first scattered beam 1011 and first reference beam 1030 ) beam pair; a sufficient phase delay between the second scattered reference beam pair including portion 1022 of the second scattered beam 1021 and the second reference beam 1040 such that any beam of one beam pair is non-identical to any beam of the other beam pair relevant. In this way, only two desired interference patterns are formed by the two scattered reference beam pairs on the image sensor 1080, respectively.

Figure 12 schematically illustrates an illumination device 1200 according to various embodiments. The irradiation apparatus of FIG. 12 is similar to the irradiation apparatus of FIG. 11 . The main difference is that in the embodiment of Figure 12, the three beamsplitters 1220, 1222, 1224 may have different split ratios compared to their respective counterparts in the embodiment of Figure 11 . A consequence of using different split ratios may be that some or all of the four radiation beams 1010, 1020, 1030, 1040 output from the device may have compared to their corresponding localized counterparts in the embodiment of Figure 11 different power. For example, in such an embodiment, the two beams 1010, 1020 output from the top branch (from beam splitter 1222) have similar powers to each other, and similarly for the two beams output from the bottom branch (from beam splitter 1224) The individual beams 1030 , 1040 have similar powers to each other, wherein the paired beams 1010 , 1020 have different powers than the paired 1030 , 1040 pair. As such, in such embodiments, the first illumination beam 1010 and the second illumination beam 1020 may be output as a pair via the beam splitter 1222, and the first reference beam 1030 and the second reference beam 1040 may be output via the beam splitter 1224 are output as a pair.

As such, Figure 11 shows an arrangement in which a first output branch (via beam splitter 1122) includes a first beam pair branch (eg, +1 diffracted order illumination and +1 reference beam), and a second output branch (via beam splitter 1122) Beam splitter 1124) includes a second beam pair branch (eg, -1 diffracted order illumination and -1 reference beam). In contrast, in Figure 12, the first output branch is the illumination branch (eg, +1 diffracted and 1st order diffracted illumination beams) and the second output branch is the reference branch (eg, +1 diffracted and 1st order diffracted beams) reference bundle).

Regardless of the different beam combinations, the three tunable optical delay lines AD1', AD2', AD3' can provide sufficient coherence (or OPD) control over all four beams so that only the desired interference pattern is formed in the image of Figure 10 sensor 1080. In an embodiment, the delay line AD3' of the reference arm may be significantly longer (tens of mm) compared to the delay line AD2' of the illumination arm.

In the arrangement of FIG. 11 , therefore, the tunable optical delay line AD1 may implement an incoherent delay arrangement operable to apply for one of the first branch or the second branch relative to the first branch or the second branch The delay of the other branch; and the adjustable optical delay lines AD2, AD3 implement coherent matching means for coherent matching of the beams within each beam pair. In contrast, in the arrangement of Figure 12, the coherent matching means and the incoherent delay arrangement can be implemented together via co-optimization between the tunable optical delay line ADl and the tunable optical delay lines AD2, AD3. It should be noted that in the latter case, the optimization would be different if prism 1131 instead of prism 1130 were adjustable, or if prism 1133 instead of prism 1134 were adjustable.

The properties of the structure of the measurement target 1060 are determined by the processing unit 1090 of the measurement device. The processing unit 1090 uses the first interference pattern and the second interference pattern recorded by the image sensor 1080 to determine the characteristics of the structure of the measurement target 1060 . In an embodiment, the processing unit 1090 is coupled to the image sensor 1080 to receive signals including information about the first interference pattern and the second interference pattern recorded by the sensor 1090 . In an embodiment, the processing unit 1090 corrects the aberrations of the objective lens 1070 of the df-DHM 1000 . In an embodiment, the measurements of the first interference pattern and the second interference pattern are performed simultaneously in time (parallel) with the radiation, and the processing unit 1090 is configured to use the measurements simultaneously in time (in parallel) to determine the measurement target 1060 Characteristics of structures on substrate 1050.

In an embodiment, the processing unit 1090 uses the first interference pattern to calculate the complex field at the sensor 1080 of the radiation associated with the portion 1012 of the first scattered radiation 1011 ("complex" here means that both amplitude information and phase information are present ). Similarly, the processing unit 1090 uses the second interference pattern to calculate the composite field at the sensor 1080 associated with the portion 1022 of the second scattered radiation 1021 . This calculation of a composite radiation field from an interference pattern formed by interfering a reference radiation with radiation scattered from an object is collectively referred to as holography. Further details on how such calculations are performed in the context of metrology for lithography can be found, for example, in US2016/0061750A1, which is hereby incorporated by reference.

If the optical properties of the df-DHM 1000 are known, it is possible to mathematically and computationally backpropagate each of the calculated complex fields to obtain the first scattered radiation at the measurement target 1060 1011 and the corresponding complex field of the second scattered radiation 1021.

The composite field is known to provide additional information for determining the characteristics of the metrology target 1060 on the substrate 1050 relative to an alternative mode in which both phase and amplitude information are unavailable . For example, in European patent application EP18158745.2, filed on 27 February 2018, it has been disclosed how to use phase information of scattered radiation to determine the overlap error between the structures of different layers on the substrate (the Examples of properties to be determined for a structure). European patent application EP18158745.2 is hereby incorporated by reference.

In an embodiment, the characteristic of the structure of the measurement target 1060 is determined by comparing the first interference pattern with the second interference pattern. In an embodiment, the properties of the structure are determined based on the difference between the first interference pattern and the second interference pattern. The difference between the first interference pattern and the second interference pattern may, for example, contain information about the asymmetry in the structure of the measurement target 1060 . Obtaining information about the asymmetry of the structure of the metrology target 1060 can provide information about overlap. In an embodiment, phase information obtained from the calculated complex field is used to obtain overlay information, as described in EP18158745.2, filed 27 February 2018. Overlap describes unwanted misalignment between different patterns in the metrology target 1060, such as patterns formed at different times, formed using different processes, and/or formed in different layers. In other embodiments, the characteristics of the structure of the metrology target 1060 being determined may include errors indicative of errors in focusing of radiation used in a lithographic process to fabricate the structures of the metrology target 1060 . In yet other embodiments, the characteristic being determined of the structure of the metrology target 1060 may include an error indicative of an error in the radiation dose of radiation used in a lithographic process to fabricate the structure of the metrology target 1060 .

It is important to minimize the contribution to the difference between the first and second interference patterns that do not originate from the structure of the measurement target 1060 (eg, the aforementioned unwanted interference patterns). By using three tunable optical delay lines AD1 , AD2 , AD3 , there is actually a pass for four radiation beams (ie, first scattered beam 1011 , first reference beam 1030 , second scattered beam 1021 , second reference beam 1040 ) ) exert sufficient coherence control to effectively suppress those contributions.

In order to accurately calculate the two composite fields, the two interference patterns should be completely separated from the background stray light and/or the residual zero diffraction order. Furthermore, in order to extract target information from each interference pattern, the two overlapping interference patterns should also be separated. Complete separation of multiple overlapping interference patterns can also be achieved using spatial frequency multiplexing. Such a method has been described in detail in US patent application US20180011022A1, which is incorporated herein by reference.

Using spatial frequency multiplexing, the processing unit 1090 subjects the recorded image including multiple overlapping interference patterns to a two-dimensional (2D) Fourier transform to obtain a Fourier transformed image. The transverse and longitudinal axes of the resulting Fourier-transformed image correspond to the two axes in the spatial frequency coordinate system (fx, fy), ie, fx and fy, respectively. In the resulting Fourier transformed image, there are multiple spatial spectra, each of which corresponds to a portion of the recorded image.

Figure 13 illustrates an exemplary 2D Fourier image in the spatial frequency domain obtained by subjecting a recorded image comprising two overlapping interference patterns to a 2D Fourier transform. As shown in FIG. 13 , the 2D Fourier image includes five spatial spectra: a fundamental spatial spectrum 1301 including a zero-order Fourier component, and the A first higher-order spatial spectrum 1311 corresponding to an interference pattern, a second higher-order spatial spectrum 1312 corresponding to the second interference pattern formed using the portion 1022 of the second scattered radiation beam 1021, and the A first conjugated spatial spectrum 1321 , which is conjugated to the first high-order spatial spectrum 1311 , and a second conjugated spatial spectrum 1322 , which is conjugated to the second high-order spatial spectrum 1312 .

The center of the fundamental spectrum is the origin O of the spatial frequency coordinates. The location of the fundamental spectrum is fixed. However, the position of the higher order spectra and their conjugate spectra can be adjusted relative to the fundamental spectrum by eg changing the angle of incidence and/or azimuth of each reference beam. The radial distance between the center of each higher-order spatial spectrum and the center of the fundamental spatial spectrum relates to the angle between the optical axis of the portion 1012 or 1022 of the scattered beam 1011 or 1021 and the optical axis of the reference beam 1030 or 1040 . The larger the angle, the further away (relative to the fundamental spatial spectrum) the higher order spatial spectrum will be. Thus, by providing a sufficiently large angle between the axis of a portion of the scattered beam and the axis of the reference beam, the higher order spatial spectrum 1311 or 1312 can be completely separated from the fundamental spatial spectrum 1301 . However, the angle of the reference beam cannot be arbitrarily high, since the increase in the angle between the optical axis of the part 1012 or 1022 of the scattered beam 1011 or 1021 and the optical axis of the reference beam 1030 or 1040 results in a decrease in the fringe spacing of the hologram fringes reduce. Ultimately, this angle is limited by the pixel pitch of the image sensor 1080 . The fringes in the hologram (or interference pattern) must be adequately sampled by the sensor pixels. The maximum frequency in the hologram must satisfy the Nyquist criterion for sampling.

Furthermore, the azimuth of each reference beam has an effect on the circumferential position of the spatial spectrum relative to the origin O. The circumferential position of the higher-order spatial spectrum is represented by the angle between the higher-order spatial spectrum and the spatial frequency axis fx. For example, the circumferential position of the first higher-order spatial spectrum is represented by angle 1331. Thus, the higher order spatial spectra 1311, 1312 can be completely separated from each other by ensuring that the difference between the azimuth angles of the two reference beams is large enough.

Once separated, the processing unit 1090 extracts each higher-order spatial spectrum from the Fourier image, and then subjects the extracted higher-order spatial spectrum to an inverse Fourier transform. It should be noted that since the two reference radiation beams are provided directly by the illuminator 1100, information (such as the intensity distribution) of the reference radiation at the image sensor 1080 can be determined by calculation or by measurement. Based on the results of the inverse Fourier transform and information on the reference radiation, a complex field of paired scattered radiation can be obtained, the details of which are described below. Ultimately, the composite field of the two scattered radiation beams is used to characterize the structure of the measurement target 1060 and/or correct for optical aberrations of the objective 1070 of the df-DHM 1000 .

Continuing to refer to Figure 13, existing methods use only the higher-order spatial spectrum or sidebands (SB) 1311 or 1312 in the 2D Fourier image 1300 for determining the amplitude of the composite field of the portion 1012 or 1022 of the scattered beam 1011 or 1021 and the phase. Information contained in the underlying spatial spectrum or central frequency band (CB) 1301 is completely discarded in the determination process. As such, existing methods are prone to noise limitations, such as low signal-to-noise ratios, thereby resulting in reduced throughput. According to various aspects of embodiments of the present invention, a method is provided that improves on existing methods by providing a better and more accurate way for determining the amplitude and phase of the complex field of scattered radiation at an image sensor. This is achieved by taking into account the information contained in both CB 1301 and SB 1311 or 1312.

It should be noted that another term for the Fourier image of Figure 13 is often referred to as the Fourier representation of the hologram. The four images of Figure 13 are obtained by transforming the hologram into its Fourier representation or Fourier spectrum in the spatial frequency domain via the (2d) Fourier transform.

14 shows a flowchart of a method for determining the amplitude and phase of a composite field (eg, the method may be performed by processing unit 1090 or otherwise), according to an embodiment. Referring to Figure 14, at step 1401, a hologram (or interference pattern) may be generated after illuminating an object or target, and then transformed via a Fourier transform into its Fourier representation or Fourier spectrum in the spatial frequency domain. This Fourier representation has the advantageous property that the CB and the corresponding SB are spatially represented in the case where the tilt angle of the reference wave used in the hologram is large enough to take into account the spatial frequency content of the SB, ie the spatial frequency content separate. It should be further noted that in Figure 13, the SBs are shown in pairs, with one pair comprising SBs 1311 and 1321 and the other pair comprising SBs 1312 and 1322. Per SB pair, two SBs carry the same information, since the two SBs are complex conjugates of each other, so that it is sufficient to select one SB per SB pair. At step 1402, a CB in the Fourier representation can be selected and used subsequently to calculate the corresponding component in the image plane via a Fourier transform of the selected CB. At step 1403, one or more individual SBs may be selected in the Fourier representation, and each of the selected SBs may be used to compute an image plane via an inverse Fourier transform of the selected SBs The corresponding components in ; at step 1404, the amplitude and/or phase of the composite field may be determined based on the calculated CB and SB components in the image plane. Details of the implementation of the method are described below.

It should be noted that the embodiment of Figure 14 is only a non-limiting example and other embodiments may include more or fewer steps as determined by specific requirements. For example, some embodiments may additionally include the step of illuminating the object or target, and may use this step as the first step; some other embodiments may combine steps 1402 and 1403 into a single step such that the inverse Fourier transform of CB and The inverse Fourier transform of SB can be performed in parallel rather than sequentially in time.

Where a single interference pattern is sufficient, one of the illumination-reference beam pair provided by illumination device 1100 or 1200 may be used to illuminate the target 1060 . Then, at step 1401, the scattered radiation from the object or target is combined with the reference radiation provided from the illumination-reference beam pair to form the desired single interference pattern. This single interference pattern can be transformed into a 2D Fourier representation in the spatial frequency domain via a Fourier transform. In such a case, a 2D Fourier representation (not shown) may include a CB and a pair of SBs that are conjugated to each other. The mutual tilt angles of the respective beams of the scattered reference beam pair may be arranged such that the resulting CB and SB do not overlap in the spatial frequency domain. Next, at step 1402, a CB in the Fourier representation may be selected and used to compute its corresponding component in the image plane via an inverse Fourier transform (ie, CB _exp as described below) (R)). Then, at step 1403, one of the mutually conjugated SBs in the Fourier representation may be selected and used to compute its corresponding component in the image plane via an inverse Fourier transform (ie, as follows SB _exp (R)) as described in the paper. Finally, at step 1404, the amplitude and phase of the complex field of scattered radiation from the object or target may be determined based on the calculated information (ie, CB _exp (R) and SB _exp (R) as described below). Step 1404 is further explained by the following mathematical description.

In the following mathematical description, the inverse Fourier transform is applied to CB and SB individually, and the corresponding components in the image plane are specified by the real-valued function CB(R) and the complex-valued function SB(R), respectively, where R is Like 2D coordinates in a plane. It should be noted that CB contains both the autocorrelation of the scattered beam as well as the autocorrelation of the reference beam. The power of the reference beam is given by: |φ _ref (R)| ² .

[1]

The complex-valued field in the image plane is denoted by _φp (R), which is equal to the sample field _φ (R) (the field scattered from the sample/target) and the imaging optics (e.g., , the convolution of the point spread function of the objective lens 1070), namely:

φ _p (R)=φ(R)*p(R).

[2]

表达为：The complex-valued field φ _p (R) in the image plane can be determined by the amplitude A(R) and the phase

Expressed as:

The hologram H(R) or interference pattern is modeled as:

where K designates the wave vector of the reference wave, and s designates the amplitude |φ _ref (R)| of the reference wave, ie s=|φ _ref (R)|.

The least squares function for the estimation of the amplitude and phase of the complex-valued field can be defined as:

S ² =∫dR(H _mod (R)-H _exp (R)) ² ,

[5]

The above least squares function can be conveniently rewritten based on Parseval's theory and the fact that CB and SB are well separated in the Fourier representation. After incorporating the corresponding contributions from CB and two conjugated SBs capable of being separated from CB, the above equation can be more explicitly expressed as:

where CB _exp (R) and SB _exp (R) designate the CB and SB components in the image plane as derived from experimentally measured holograms, respectively, and the modeled CB and SB components, i.e.

CB _mod (R) and SB _mod (R) are expressed as:

CB _mod (R)=s ² +(1-s) ² A(R) ² ,

[7]

and

For simplicity, CB _exp (R) and SB _exp (R) will be referred to as experimentally measured CB and SB components.

的参数拟合：Obtained for amplitude A(R) and phase via the corresponding derivatives of S2, ie equation [ ⁶ ]

The parametric fit of :

^The latter derivative of S2, equation [10] yields (for a particular value of R):

估计为：From the above equation, the phase of the composite field can be

Estimated to be:

因此相位

的估计值不依赖于参考束的振幅|φ_ref(R)|或因子s。然而，对于边频带中的最优信噪比，s的值可以被选择为s＝0.5。Since it is possible to measure the phase based only on the sidebands

so the phase

The estimate of is independent of the reference beam amplitude |φ _ref (R)| or the factor s. However, for optimal signal-to-noise ratio in the sidebands, the value of s can be chosen to be s=0.5.

^The previous derivative of S2, equation [9] yields (for any value of position in the image plane R):

The above relationship can be simplified to (ignoring explicit R dependencies):

In this relationship, the first term involves CB, and the second term involves SB. For the following line of inference, then the resolution of the amplitude for each of these two terms can be obtained individually, which results at CB as the corresponding estimate for the amplitude:

and at SB:

where ^ indicates an estimate.

Using such two expressions, equations [15] and [16], equation [14] resulting from the derivative of S2 with respect to ^A can be simplified to:

The true resolution (positive, and real-valued) of the amplitude A at any image location R can be easily solved from the above cubic equation using standard mathematical methods [17]. Some properties of interest for true resolution can be further derived. According to the fact that A>0 and 0<s<1, the two schemes can be regarded as either of the following:

or vice versa:

Expressed differently, this also implies either of the following:

or vice versa:

In situations where two (overlapping) interference patterns are required (such as the operation described with respect to the embodiment of FIG. 10 ), both the illumination-reference beam pair provided by the illumination device 1100 or 1200 may be used to illuminate the target 1060. As described above, the two (overlapping) interference patterns may be formed from two pairs of scattered reference beams, respectively. Therefore, the processing unit will need to determine the amplitude and phase of the two composite fields, one for each scattered beam. In some embodiments, the processing unit 1070 may also adopt the above four steps (ie, 1401 to 1404 in FIG. 14 ) to complete the determination process. In this case, however, the 2D Fourier representation may include a superposition of two corresponding CBs on top of each other, and two pairs of SBs (or four SBs in total) that are mutually conjugated and well-separated, such as, for example, the example of Figure 13 Fourier said. Each pair of mutually conjugated SBs contains information about one of the two interference patterns. For example, the first pair of mutually conjugated SBs contains information about the first interference pattern formed by the first pair of scattered reference beams; and the second pair of mutually conjugated SBs contains information about the first interference pattern formed by the second pair of scattered reference beams. Information on two interference patterns. The two scattered reference beam pairs may be arranged such that the resulting CB and SB do not overlap or overlap in the spatial frequency domain.

Following step 1401 of Fourier transforming the captured image of the interference pattern to the spatial frequency domain, at step 1402 a central region in the Fourier representation comprising two overlapping corresponding CBs is selected and the center The region is used to calculate its corresponding component, ie CB _exp (R) in the image plane via an inverse Fourier transform. It should be noted that for ease of notation, CB _exp (R) here represents the superimposed CB of the two corresponding scattered reference beam pairs. The same notation will be used for the modeled version of this component. Then, at step 1403, one SB of each pair of SBs in the Fourier representation is selected and used to compute its corresponding component in the image plane via an inverse Fourier transform. Thus, two image plane components SB _1,exp (R) and SB _2,exp (R) can be obtained with i.e. indices "1" and "2", where these indices refer to two different interference patterns (or composite fields) ). Finally, at step 1404, the amplitude and phase of the two complex fields can be determined based on the calculated information, eg, CB _exp (R), SB1 _,exp (R), and SB2 _,exp (R). Step 1404 is further explained by the following mathematical description, which is an extension of the previous mathematical description (equations [1] to [21]) for a single hologram or interference pattern in this case.

In the following mathematical description, an inverse Fourier transform is applied to the CB of Figure 13 (eg 1301) and each of the two selected SBs in the Fourier image (eg 1311 and 1312 of Figure 13), respectively, in order to obtain the corresponding components in the image plane. Such image plane components are represented by real-valued functions CB(R) and complex-valued functions SB ₁ (R) and SB ₂ (R), respectively, where R is a 2D coordinate in the image plane and indices "1" and "2" Refers to two different interference patterns (or composite fields). Without loss of generality, the multiplexed hologram H(R) is modeled as an incoherent superposition of two separate holograms or interference patterns ( where the indices "1" and "2" refer to two separate holograms):

The Fourier transform of the above multiplexed hologram contains a single CB in the Fourier representation (which originates from two separate CBs and two separate SBs of two corresponding scattered reference beam pairs, which and two separate SBs are modeled as follows:

CB _mod (R)=s ² +(1-s) ² A ₁ (R) ² +s ² +(1-s) ² A ₂ (R) ² , [23]

and

The least squares function to be minimized for the multiplexed hologram is:

和

的优化涉及求取S²的相对于每个相位函数

或

的导数且因此对于非复用(单个)全息图的情况是相同的，这是由于相位仅在相应的SB中是能够检测到的(并且在针对两个相应的散射参考束发生重叠的CB中则不是)。这种情形意味着，关于单独的全息图的相位将导出即得到相同分辨率(这是由于它们的边频带在复用全息图的傅里叶空间中被分离)：for phase

and

^The optimization involves finding the phase function of S2 with respect to each

or

and is therefore the same for the case of non-multiplexed (single) holograms, since the phase is only detectable in the corresponding SB (and in the CB where the overlap occurs for the two corresponding scattered reference beams) is not). This situation means that the phases with respect to the individual holograms will be derived to give the same resolution (since their sidebands are separated in the Fourier space of the multiplexed hologram):

and

Similarly, optimization for amplitudes A ₁ (R) and A ₂ (R) involves taking the derivative of S ² with respect to each amplitude function A ₁ (R) or A ₂ (R), and at A ₁ (R) ) and A ₂ (R) yield the following two equations:

和

以上两个方程[29]和[30]可以被重写为：Apply Relational Expressions

and

The above two equations [29] and [30] can be rewritten as:

The above system of equations, ie, simultaneous equations, can be solved via various strategies. Without loss of generality, a particular strategy is described herein as an example of implementation. The ratio of the two amplitudes is obtained by dividing the two equations [31] and [32]:

Applying Equation [33] to Equation [31] in order to remove A ₂ (R), the equation with a unique unknown parameter A ₁ (R) can be obtained (explicit R dependence omitted):

where [epsilon] is a small positive value that avoids noise amplification in regions of low power in the SB signal. The true solution of the (positive and real-valued) amplitude A ₁ of the first hologram at a particular image location R contained in the multiplexed hologram can be easily solved according to the cubic equation [34]. The value of the amplitude A2 of the _second hologram contained in the multiplexed hologram can then be derived from the relation of the ratio, ie equation [33].

It should be noted that the foregoing embodiments may also be further generalized or generalized for df-DHMs with multiple pairs of illumination and reference radiation beams. In some embodiments, each of the plurality of illuminating radiation beams may include different azimuth angles and/or different angles of incidence. Likewise, each of the plurality of reference beams may include a different azimuth angle and/or a different angle of incidence. For example, in an embodiment, in addition to the two illuminating radiation beams 1010 , 1020 located primarily in the x-z plane, two other illuminating radiation beams (not shown) located primarily in the y-z plane may be used to illuminate the target 1060 . Two additional reference radiation beams may also be used to pair with two additional illuminating radiation beams, respectively. This situation may result in four at least partially spatially overlapping interference patterns, each of which may correspond to a pair of illumination and reference radiation beams. By appropriately configuring the azimuth and/or incidence angle of each additional reference radiation beam, the four at least partially spatially overlapping interference patterns may be separable in the spatial frequency domain. In this way, more information about the structure of the target (eg y-axis asymmetry in the structure of the target) can be obtained.

In some embodiments, the illumination apparatus may provide multiple pairs of illumination radiation beams and reference radiation beams. The illumination device may also provide sufficient coherence control between the radiation beams so that only the desired interference pattern will be formed on the image sensor. The plurality of pairs of the illuminating radiation beam and the reference radiation beam may result in the formation of a plurality of mutually incoherent, spatially overlapping interference patterns. Thus, the multiplexed hologram H(R) can be modeled as an incoherent superposition of multiple individual holograms or interference patterns. Equation [22] for the case of two overlapping holograms can then be further extended to include the amplitude and phase functions of all individual holograms. Therefore, when n holograms are formed, the hologram index, ie, the index, should expand from "1,2" to "1,2,3...and n". It should be noted that the determination process as illustrated in Figure 14 should be equally applicable to any number of overlapping holograms, such as more than two overlapping holograms.

It should also be noted that different embodiments of the method for determining the amplitude and phase of one or more complex fields (eg, the embodiment of FIG. 14 ) may be used in combination with the embodiments of FIGS. The embodiments of 10 to 12 are used independently. Other types of df-DHMs can be used to generate holograms or interference patterns when used independently, ie independently of each other.

In an embodiment, the processing unit 1090 may be a computer system. The computer system may be equipped with an image reconstruction algorithm to perform all of the aforementioned tasks, including performing Fourier transforms, extracting each individual higher-order spatial spectrum, performing inverse Fourier transforms, computing complex fields, and based on the results The properties of the structure are determined.

15 is a block diagram illustrating a computer system 1500 that can assist in implementing the methods and processes disclosed herein. Computer system 1500 includes a bus 1502 or other communication mechanism for communicating information, and a processor 1504 (or multiple processors 1504 and 1505) coupled with bus 1502 for processing information. Computer system 1500 also includes main memory 1506, such as random access memory (RAM) or other dynamic storage, coupled to bus 1502 for storing information and instructions to be executed by processor 1504. Main memory 1506 may also be used to store transient variables or other intermediate information during execution of instructions to be executed by processor 1504 . Computer system 1500 also includes a read only memory (ROM) 1508 or other static storage device coupled to bus 1502 for storing static information and instructions for processor 1504 . A storage device 1510, such as a magnetic or optical disk, is provided and coupled to bus 1502 for storing information and instructions.

Computer system 1500 may be coupled by bus 1502 to a display 1512 for displaying information to a computer user, such as a cathode ray tube (CRT) or flat panel display or touch panel display. An input device 1514 , including alphanumeric keys and other keys, is coupled to the bus 1502 for communicating information and command selections to the processor 1504 . Another type of user input device is cursor control 1516, such as a mouse, trackball, or cursor directional keys, for communicating directional information and command selections to processor 1504 and for controlling cursor movement on display 1512. Such input devices typically have two degrees of freedom in two axes, a first axis (eg, x) and a second axis (eg, y), which allows the device to specify a position in a plane. A touch panel (screen) display can also be used as an input device.

Method or methods as described herein may be performed by computer system 1500 in response to processor 1504 executing one or more sequences of one or more instructions contained in main memory 1506 . These instructions may be read into main memory 1506 from another computer-readable medium, such as storage device 1510 . Execution of the sequences of instructions contained in main memory 1506 causes processor 1504 to perform the process steps described herein. One or more processors in a multiprocessing arrangement may also be used to execute sequences of instructions contained in main memory 1506 . In alternative embodiments, hardwired circuitry may be used in place of or in combination with software instructions. Accordingly, the descriptions herein are not limited to any specific combination of hardware circuitry and software.

The term "computer-readable medium" as used herein refers to any medium that participates in providing instructions to processor 1504 for execution. Such a medium may take many forms, including but not limited to non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 1510 . Volatile media includes volatile memory, such as main memory 1506 . Transmission media includes coaxial cables, copper wire, and fiber optics, including the wires that comprise bus 1502. Transmission media can also take the form of acoustic or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer readable media include, for example, floppy disks, flexible disks, hard disks, magnetic tapes, any other magnetic media, CD-ROMs, DVDs, any other optical media, punched cards, paper tape, having a pattern of holes , RAM, PROM and EPROM, FLASH-EPROM, any other memory chip or cartridge, carrier wave as described below, or any other medium that can be read by a computer.

Various forms of computer-readable media may be involved in carrying one or more sequences of one or more instructions to processor 1504 for execution. For example, the instructions may initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions into its volatile memory and send the instructions over a telephone line using a modem. A modem local to computer system 1500 can receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector coupled to bus 1502 can receive the data carried in the infrared signal and place the data on bus 1502 . Bus 1502 carries data to main memory 1506 from which processor 1504 retrieves and executes instructions. Instructions received by main memory 1506 may optionally be stored on storage device 1510 either before or after execution by processor 1504 .

Computer system 1500 also preferably includes a communication interface 1518 coupled to bus 1502 . Communication interface 1518 provides a two-way data communication coupling to network link 1520 , which connects to local area network 1522 . For example, communication interface 1518 may be an Integrated Services Digital Network (ISDN) card or modem to provide a data communication connection to a corresponding type of telephone line. As another example, the communication interface 1518 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links can also be implemented. In any such implementation, communication interface 1518 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

Network link 1520 typically provides data communications to other data devices via one or more networks. For example, network link 1520 may provide a connection from local area network 1522 to host computer 1524 or to data equipment operated by Internet Service Provider (ISP) 1526. The ISP 1526, in turn, provides data communication services via a global packet data communication network (now commonly referred to as the "Internet" 1528). Both the local area network 1522 and the Internet 1528 use electrical, electromagnetic or optical signals that carry digital data streams. The signals via the various networks and the signals on network link 1520 and via communication interface 1518, which carry digital data to and from computer system 1500, are exemplary forms of carrier waves that carry the information.

Computer system 1500 can send messages and receive data, including program code, over a network, network link 1520, and communication interface 1518. In the Internet example, the server 1530 may transmit the requested code for the application program via the Internet 1528, the ISP 1526, the local area network 1522, and the communication interface 1518. For example, one such downloaded application may provide one or more of the techniques described herein. The received code may be executed by the processor 1504 as it is received, and/or stored in the storage device 1510 or other non-volatile storage for later execution. In this manner, computer system 1500 may obtain application code in the form of a carrier wave.

Additional embodiments are disclosed in the subsequent numbered lists of aspects:

1. A dark-field digital holographic microscope configured to determine properties of interest of a structure, the dark-field digital holographic microscope comprising:

an illumination device configured to provide at least: a first beam pair comprising a first illumination radiation beam and a first reference radiation beam; and a second beam pair comprising a second beam of illumination radiation and a second beam of reference radiation; and

an imaging branch operable to detect at least a first scattered radiation scattered by the structure and a second scattered radiation scattered by the structure, the first scattered radiation being affected by the first scattered radiation by the structure produced by irradiation with a beam of illuminating radiation, the second scattered radiation produced by the structure being irradiated with the second beam of irradiation radiation, the imaging branch having a detection NA greater than 0.1 and optionally greater than 0.8;

wherein the illumination device is configured such that:

the first beam of illumination radiation and the first beam of reference radiation are at least partially temporally coherent and at least partially spatially coherent;

the second beam of illumination radiation and the second beam of reference radiation are at least partially temporally coherent and at least partially spatially coherent; and

The illumination device is configured to impose temporal and/or spatial incoherence between the first beam pair and the second beam pair.

2. The darkfield digital holographic microscope of clause 1, wherein the illumination device is operable to: direct the first beam of illumination radiation to illuminate the structure from a first direction; and direct the second beam of illumination radiation so that the structure is illuminated from a second direction, the second direction being different from the first direction.

3. The darkfield digital holographic microscope of aspect 1 or 2, wherein the imaging branch comprises a sensor, and the darkfield digital holographic microscope is operable to simultaneously capture an interference image on the sensor, the interference image comprising A first interference pattern resulting from the interference of the first scattered radiation with the first reference beam, and a second interference pattern resulting from the interference of the second scattered radiation with the second reference beam.

4. The darkfield digital holographic microscope of clause 3, operable such that the first interference pattern and the second interference pattern at least partially spatially intersect on the sensor stack.

5. The darkfield digital holographic microscope of clause 3 or 4, the darkfield digital holographic microscope being configured such that the first reference radiation beam and the second reference radiation beam are each arranged relative to the The optical axis of the dark-field digital holographic microscope is incident at corresponding different azimuth angles.

6. The darkfield digital holographic microscope of clause 5, wherein the azimuth angle of the first reference radiation beam and the azimuth angle of the second reference radiation beam are configured to include a sufficiently large difference such that The two interference patterns are separable in the spatial frequency domain.

7. The darkfield digital holographic microscope of any one of aspects 3 to 6, the darkfield digital holographic microscope being configured such that the first reference radiation beam and the second reference radiation beam are arranged opposite each other The optical axis of the dark-field digital holographic microscope is incident at correspondingly different incident angles.

8. The darkfield digital holographic microscope of any one of aspects 3 to 7, comprising a processor operable to:

transforming the interference images of the first interference pattern and the second interference pattern into a Fourier representation, wherein the Fourier representation includes a center band and at least one pair of sidebands; and

The amplitude of the composite field of at least one of the first scattered radiation and the second scattered radiation is determined from the center frequency band and at least one sideband of the at least one pair of sidebands.

9. The darkfield digital holographic microscope of clause 8, wherein the at least one pair of sidebands comprises:

a first pair of conjugate sidebands, the first pair of conjugate sidebands including first information related to the first interference pattern of the scattered reference beam pair, and

A second pair of conjugate sidebands includes second information related to the second interference pattern of the scattered reference beam pair.

10. The darkfield digital holographic microscope of clause 9, wherein the processor is operable to:

using the central frequency band to calculate a first component in the interference image of the first interference pattern and the second interference pattern via an inverse Fourier transform;

using the first sideband to calculate a second component in the interference image of the first interference pattern and the second interference pattern via an inverse Fourier transform;

using the second sideband to calculate a third component in the interference image of the first interference pattern and the second interference pattern via an inverse Fourier transform; and

A first amplitude and a first phase of a first complex field of the first scattered radiation are determined from the first, second and third components of the interference image, and the second A second amplitude and a second phase of the second composite field of scattered radiation.

11. The darkfield digital holographic microscope of clause 10, wherein the processor is configured such that the determination of a first amplitude and a first phase of the composite field of the first scattered radiation and the determination of the Determining the second amplitude and second phase of the composite field of the second scattered radiation further comprises:

obtaining modeled images of the first interference pattern and the second interference pattern, the modeled images including a modeled first component, a modeled second component, and a modeled third component;

defining a performance function describing the matching of the interference image and the modeled image; and

optimizing (eg, minimizing) the performance function to obtain values for one or more of: the first phase, the second phase, the first amplitude, and the second amplitude.

12. The darkfield digital holographic microscope of clause 11, wherein the processor is configured such that the optimization of the performance function further comprises:

fitting the value of the first phase by obtaining a derivative of the performance function with respect to the first phase;

fitting the value of the second phase by obtaining a derivative of the performance function with respect to the second phase;

fitting the value of the first amplitude by obtaining a derivative of the performance function with respect to the first amplitude; and

The value of the second amplitude is fitted by obtaining a derivative of the performance function with respect to the second amplitude.

13. A darkfield digital holographic microscope according to any preceding aspect, wherein the illumination means further comprises coherent matching means operable to adjustably combine the first illumination beam and all delaying one of the first reference beams relative to the other of the first illumination beam and the first reference beam to maintain the beams of the first pair of beams at least partially coherent; and with delaying one of the second illumination beam and the second reference beam relative to the other of the second illumination beam and the second reference beam in an adjustable manner so as to delay the second beam Pairs of beams remain at least partially coherent.

14. The darkfield digital holographic microscope of clause 13, wherein the illumination means comprises a time delay means configured to pass the first beam pair and the second beam pair by being operable to One of the first beam pair and the second beam pair is delayed relative to the other of the first beam pair and the second beam pair to impose incoherence between the first beam pair and the second beam pair.

15. The darkfield digital holographic microscope of clause 14, wherein the time delay means comprises an adjustable time delay means operable to adjust the first beam pair and all One of the second beam pair is delayed relative to the other of the first beam pair and the second beam pair to impose the incoherence.

16. A darkfield digital holographic microscope according to clause 14 or 15, wherein the illumination means comprises a first branch operable to provide the first pair of beams and a second branch operable to provide the second pair of beams branch, and where:

The time delay means includes at least a delay line operable to impose on one of the first branch or the second branch relative to the other of the first branch or the second branch delay; and

The coherent matching means includes first coherent matching means in the first branch operable to adjustably delay the first reference beam and the first illumination beam at least one of; and second coherent matching means in said second branch operable to adjustably delay at least one of said second reference beam and said second illumination beam One.

17. The darkfield digital holographic microscope of clause 14 or 15, wherein the illumination means comprises: a first branch operable to provide the first illumination beam and the second illumination beam; and a second branch operable to provide the first reference beam and the second reference beam; wherein:

The coherent matching means and the time delay means are implemented via co-optimization of at least a first adjustable delay line operable to pair the first branch or the One of the second branches imposes an adjustable delay with respect to the first branch or the other of the second branches; a second adjustable delay line in the first branch, the second adjustable delay a line operable to impose an adjustable delay on at least one of the first illumination beam and the second illumination beam; and a third adjustable delay line in the second branch, the third adjustable delay The wire is operable to impose an adjustable delay on at least one of the first reference beam and the second reference beam.

18. The darkfield digital holographic microscope of any preceding aspect, wherein the first beam of illumination radiation is configured to illuminate the structure at a first angle of incidence; the second beam of illumination radiation is configured to illuminate the structure at a different A second angle of incidence of the first angle of incidence illuminates the structure.

19. A darkfield digital holographic microscope according to any preceding aspect, wherein the illumination device comprises a single radiation source, the illumination device being configured to generate the first beam pair and the second beam from the single radiation source right.

20. The darkfield digital holographic microscope of clause 19, wherein the single light source is configured to emit at least partially coherent radiation.

21. A darkfield digital holographic microscope according to any preceding aspect, wherein the illumination device is configured to generate the first reference beam and the second reference beam, respectively, at a first power level, and at a second power The first and second illumination beams are generated at levels, respectively, the second power level being greater than the first power level.

22. The darkfield digital holographic microscope of any preceding aspect, further comprising: one or more optical elements operable to capture the first scattering scattered by the structure radiation, the first scattered radiation resulting from the structure being irradiated by the first illuminating radiation beam; and to capture second scattered radiation scattered by the structure, the second scattered radiation being irradiated by the structure The second irradiating radiation beam is irradiated.

23. A darkfield digital holographic microscope according to any preceding aspect, wherein the imaging branch further comprises an objective lens operable to capture at least the first scattered radiation and the second scattered radiation.

24. The darkfield digital holographic microscope of any preceding aspect, wherein the imaging branch comprises a net positive magnification.

25. A darkfield digital holographic microscope according to any preceding aspect, wherein the illumination device is configured such that the first illumination beam and the second illumination beam each comprise a smooth profile so as to illuminate substantially uniformly the structure.

26. A method of determining a characteristic of interest of a target formed on a substrate by a photolithographic process, the method comprising:

irradiating the target with a first beam of illumination radiation and capturing the resulting first scattered radiation that has scattered from the target;

irradiating the target with a second beam of illumination radiation and capturing the resulting second scattered radiation that has been scattered from the target;

impose spatial incoherence and/or between a first beam pair comprising the first illumination beam and the first reference beam and a second beam pair comprising the second illumination beam and the second reference beam Time incoherence such that:

the beams of the first beam pair are at least partially spatially coherent and at least partially temporally coherent,

the beams of the second beam pair are at least partially spatially coherent and at least partially temporally coherent, and

any beam of the first beam pair is spatially incoherent and/or temporally incoherent with respect to any beam of the second beam pair; and

A first interference pattern resulting from interference of the first scattered radiation with a first reference beam of radiation and a second interference pattern resulting from interference of the second scattered radiation with a second reference beam are simultaneously generated.

27. The method of aspect 26, further comprising:

directing the first beam of illumination radiation to illuminate the target at a first angle of incidence; and directing the second beam of illumination radiation to illuminate the target at a second angle of incidence, the first angle of incidence being different from the second angle of incidence horn.

28. The method according to any one of aspects 26 or 27, further comprising:

directing the first beam of illuminating radiation to illuminate the target at a first azimuth angle; and directing the second beam of illuminating radiation to illuminate the target at a second azimuth angle, the first azimuth angle being different from the second azimuth horn.

29. The method of any one of aspects 26 to 28, further comprising:

The first reference radiation beam and the second reference radiation beam are directed to be incident at respective different azimuthal angles with respect to the optical axis of the darkfield digital holographic microscope.

30. The method of clause 29, wherein the azimuth angle of the first reference beam of radiation and the azimuth angle of the second reference beam of radiation comprise a sufficiently large difference such that two of the interference patterns are in space. can be separated in the frequency domain.

31. The method according to any one of aspects 29 or 30, further comprising:

The first reference radiation beam and the second reference radiation beam are directed to be incident at respective different angles of incidence with respect to the optical axis of the darkfield digital holographic microscope.

32. The method of any one of aspects 26 to 31, further comprising:

Adjustable delaying of one of the first illumination beam and the first reference beam relative to the other of the first illumination beam and the first reference beam to delay the first illumination beam and the first reference beam maintaining beams of a beam pair at least partially coherent; and adjusting one of the second illumination beam and the second reference beam relative to the second illumination beam and the second reference beam The other of the beams is delayed in order to maintain the beams of the second beam pair at least partially coherent.

33. The method of aspect 32, further comprising:

Adjustable delay of one of the first beam pair and the second beam pair relative to the other of the first beam pair and the second beam pair so that the first beam pair and the second beam pair are delayed Incoherence is imposed between the pair of beams and the second pair of beams.

34. The method of any one of aspects 26 to 33, further comprising:

The first beam pair and the second beam pair are generated from a common radiation source, the first beam pair including the first illumination radiation beam and the first reference radiation beam, and the second beam pair including the second beam of illumination radiation and the second beam of reference radiation.

35. The method of any one of aspects 26 to 34, further comprising:

Setting the first reference beam and the second reference beam to a first power level and setting the first illumination beam and the second illumination beam to a second power level, the second power level being greater than the second power level a power level.

36. The method according to any one of aspects 26 to 35, comprising maintaining the first beam pair and the second beam pair when the incoherence is imposed between the first beam pair and the second beam pair. The time delay between the second beam pair is as short as possible.

37. The method of any one of aspects 26 to 36, further comprising:

The first interference pattern and the second interference pattern are imaged such that the first interference pattern and the second interference pattern at least partially spatially overlap to obtain an interference image.

38. The method of aspect 37, comprising the additional steps of:

transforming the interference images of the first interference pattern and the second interference pattern into a Fourier representation, wherein the Fourier representation includes a center band and at least one pair of sidebands; and

An amplitude of a composite field of at least one of the first scattered radiation and the second scattered radiation is determined from the center band and the at least one sideband of the at least one pair of sidebands.

39. The method of clause 38, wherein the at least one pair of sidebands comprises:

a first pair of conjugate sidebands, the first pair of conjugate sidebands including first information related to the first interference pattern, and

A second pair of conjugate sidebands includes second information related to the second interference pattern.

40. The method of clause 39, wherein the determining step further comprises:

using the central frequency band to calculate, via an inverse Fourier transform, a first component in the interference image of the first interference pattern and the second interference pattern;

using the first sideband to calculate a second component in the interference image of the first interference pattern and the second interference pattern via an inverse Fourier transform;

using the second sideband to calculate a third component in the image of the first interference pattern and the second interference pattern via an inverse Fourier transform; and

A first amplitude and a first phase of a first complex field of the first scattered radiation are determined from the first, second and third components of the interference image, and the second A second amplitude and a second phase of the second composite field of scattered radiation.

41. The method of clause 40, wherein each of the first pair of conjugate sidebands and the second pair of conjugate sidebands is separable from the center band and any other sidebands.

42. The method of clause 40 or 41, wherein the determination of a first amplitude and a first phase of the complex field of the first scattered radiation and the complex field of the second scattered radiation The determination of the second amplitude and the second phase also includes:

obtaining modeled images of the first interference pattern and the second interference pattern, the modeled images including a modeled first component, a modeled second component, and a modeled third component;

defining a performance function describing a match between the interference image and the modeled image; and

optimizing (eg, minimizing) the performance function to obtain values for one or more of: the first phase, the second phase, the first amplitude, and the second amplitude.

43. The method of clause 42, wherein the optimizing of the performance function further comprises:

fitting the value of the first phase by obtaining a derivative of the performance function with respect to the first phase;

fitting the value of the second phase by obtaining a derivative of the performance function with respect to the second phase;

fitting the value of the first amplitude by obtaining a derivative of the performance function with respect to the first amplitude;

The value of the second amplitude is fitted by obtaining a derivative of the performance function with respect to the second amplitude.

44. A metrology apparatus for determining a property of interest of a structure on a substrate, said metrology apparatus comprising a darkfield digital holography according to any of aspects 1 to 22; or any of aspects 52 to 56 microscope.

45. An inspection apparatus for inspecting structures on a substrate, the inspection apparatus comprising a darkfield digital holographic microscope according to any one of aspects 1 to 25; or aspects 52 to 56.

46. A method of determining at least the amplitude of a complex field describing a structure, the method comprising:

irradiating the structure with a first beam of illumination radiation and capturing the resulting first scattered radiation that has scattered from the structure;

irradiating the structure with a second beam of illumination radiation and capturing the resulting second scattered radiation that has scattered from the structure;

imaging a first interference pattern resulting from interference of the first scattered radiation with a first reference beam of radiation and a second interference pattern resulting from interference of the second scattered radiation with a second reference beam such that the the first interference pattern and the second interference pattern at least partially spatially overlap to obtain an interference image;

transforming the interference images of the first interference pattern and the second interference pattern into a Fourier representation, wherein the Fourier representation includes a center band and at least one pair of sidebands; and

An amplitude of a composite field of at least one of the first scattered radiation and the second scattered radiation is determined from the center frequency band and the at least one pair of sidebands.

47. The method of clause 46, wherein the at least one pair of sidebands comprises:

a first pair of conjugate sidebands, the first pair of conjugate sidebands including first information related to the first interference pattern, and

A second pair of conjugate sidebands includes second information related to the second interference pattern.

48. The method of clause 47, wherein the determining step further comprises:

using the central frequency band to calculate a first component in the interference image of the first interference pattern and the second interference pattern via an inverse Fourier transform;

using the first sideband to calculate a second component in the interference image of the first interference pattern and the second interference pattern via an inverse Fourier transform;

using the second sideband to calculate a third component in the interference image of the first interference pattern and the second interference pattern via an inverse Fourier transform; and

A first amplitude and a first phase of a first complex field of the first scattered radiation are determined from the first, second and third components of the interference image, and the second A second amplitude and a second phase of the second composite field of scattered radiation.

49. The method of clause 48, wherein each of the first pair of conjugate sidebands and the second pair of conjugate sidebands is separable from the center band and any other sidebands.

50. The method of clause 48 or 49, wherein the determination of a first amplitude and a first phase of the complex field of the first scattered radiation and the complex field of the second scattered radiation The determination of the second amplitude and the second phase also includes:

obtaining modeled images of the first interference pattern and the second interference pattern, the modeled images including a modeled first component, a modeled second component, and a modeled third component;

defining a performance function describing a match between the interference image and the modeled image; and

optimizing (eg, minimizing) the performance function to obtain values for one or more of: the first phase, the second phase, the first amplitude, and the second amplitude.

51. The method of clause 50, wherein the optimizing of the performance function further comprises:

fitting the value of the first phase by obtaining a derivative of the performance function with respect to the first phase;

fitting the value of the second phase by obtaining a derivative of the performance function with respect to the second phase;

fitting the value of the first amplitude by obtaining a derivative of the performance function with respect to the first amplitude;

The value of the second amplitude is fitted by obtaining a derivative of the performance function with respect to the second amplitude.

52. A darkfield digital holographic microscope configured to determine properties of interest of a structure, the darkfield digital holographic microscope comprising:

an illumination apparatus configured (eg, simultaneously) to provide at least: a first beam pair comprising a first illumination radiation beam and a first reference radiation beam; and a second beam pair, the The second beam pair includes a second beam of illumination radiation and a second beam of reference radiation, such that the dark-field digital holographic microscope is operable to (eg, simultaneously) capture radiation scattered by the structure affected by the first beam. first scattered radiation generated by irradiation with an illuminating radiation beam; and capturing second scattered radiation scattered by the structure resulting from irradiating the structure with the second illuminating radiation beam;

a sensor operable to simultaneously capture an interference image including a first interference pattern resulting from interference of the first scattered radiation with the first reference beam and by the second scattered radiation a second interference pattern resulting from interference with the second reference beam; and

a processor operable to:

transforming the interference images of the first interference pattern and the second interference pattern into a Fourier representation, wherein the Fourier representation includes a center band and at least one pair of sidebands; and

An amplitude of a composite field of at least one of the first scattered radiation and the second scattered radiation is determined from the center frequency band and the at least one pair of sidebands.

53. The darkfield digital holographic microscope of clause 52, wherein the at least one pair of sidebands comprises:

a first pair of conjugate sidebands, the first pair of conjugate sidebands including first information related to the first interference pattern, and

A second pair of conjugate sidebands includes second information related to the second interference pattern.

54. The darkfield digital holographic microscope of clause 53, wherein the processor is operable to:

using the central frequency band to calculate a first component in the interference image of the first interference pattern and the second interference pattern via an inverse Fourier transform;

using the first sideband to calculate a second component in the interference image of the first interference pattern and the second interference pattern via an inverse Fourier transform;

using the second sideband to calculate a third component in the interference image of the first interference pattern and the second interference pattern via an inverse Fourier transform; and

A first amplitude and a first phase of a first complex field of the first scattered radiation are determined from the first, second and third components of the interference image, and the second A second amplitude and a second phase of the second composite field of scattered radiation.

55. The darkfield digital holographic microscope of clause 54, wherein the processor is configured such that the determination of a first amplitude and a first phase of the composite field of the first scattered radiation and the determination of the Determining the second amplitude and second phase of the composite field of the second scattered radiation further comprises:

obtaining modeled images of the first interference pattern and the second interference pattern, the modeled images including a modeled first component, a modeled second component, and a modeled third component;

defining a performance function describing a match between the interference image and the modeled image; and

optimizing (eg, minimizing) the performance function to obtain values for one or more of: the first phase, the second phase, the first amplitude, and the second amplitude.

56. The darkfield digital holographic microscope of clause 55, wherein the processor is configured such that the optimization of the performance function further comprises:

fitting the value of the first phase by obtaining a derivative of the performance function with respect to the first phase;

fitting the value of the second phase by obtaining a derivative of the performance function with respect to the second phase;

fitting the value of the first amplitude by obtaining a derivative of the performance function with respect to the first amplitude; and

The value of the second amplitude is fitted by obtaining a derivative of the performance function with respect to the second amplitude.

While specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the fabrication of integrated optical systems, guidance and detection patterns for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

Although specific reference may be made herein to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatuses. Embodiments of the invention may form part of mask inspection equipment, metrology equipment, or any equipment that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatuses may generally be referred to as lithographic tools. Such lithography tools can use vacuum conditions or ambient (non-vacuum) conditions.

While specific reference may be made above to the use of embodiments of the invention in the context of photolithography, it should be understood that the invention is not limited to photolithography and may be used for other applications (eg, as the context allows) , imprint lithography).

While specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced otherwise than as described. The above description is intended to be illustrative, not restrictive. Accordingly, those skilled in the art will appreciate that modifications may be made to the invention as described without departing from the scope of the claims set forth below.

Claims (15)

1. A dark-field digital holographic microscope configured to determine a property of interest of a structure, the dark-field digital holographic microscope comprising:

an illumination device configured to provide at least: a first beam pair comprising a first illumination radiation beam and a first reference radiation beam; and a second beam pair comprising a second illumination radiation beam and a second reference radiation beam; and

an imaging branch operable to capture at least a first scattered radiation scattered by the structure resulting from illumination of the structure by the first illumination radiation beam and a second scattered radiation scattered by the structure resulting from illumination of the structure by the second illumination radiation beam, the imaging branch having a detection NA greater than 0.1;

wherein the illumination device is configured such that:

the first illumination radiation beam and the first reference radiation beam are at least partially temporally coherent and at least partially spatially coherent;

the second illumination radiation beam and the second reference radiation beam are at least partially temporally coherent and at least partially spatially coherent; and

the illumination device is configured to impose temporal and/or spatial incoherence between the first beam pair and the second beam pair.

2. The dark field digital holographic microscope of claim 1, wherein the illumination device is operable to: directing the first beam of illumination radiation to illuminate the structure from a first direction; and directing the second beam of illumination radiation to illuminate the structure from a second direction, the second direction being different from the first direction.

3. The dark-field digital holographic microscope of claim 1 or 2, wherein the imaging branch further comprises a sensor and the dark-field digital holographic microscope is operable to simultaneously capture on the sensor a first interference pattern resulting from interference of the first scattered radiation with the first reference beam and a second interference pattern resulting from interference of the second scattered radiation with the second reference beam, wherein optionally the dark-field digital holographic microscope is operable such that the first and second interference patterns at least partially spatially overlap on the sensor.

4. The dark-field digital holographic microscope according to claim 3, configured such that the first reference radiation beam and the second reference radiation beam are arranged to each be incident at respective different azimuth angles with respect to an optical axis of the dark-field digital holographic microscope, wherein optionally the azimuth angle of the first reference radiation beam and the azimuth angle of the second reference radiation beam are configured to comprise a difference large enough such that the two interference patterns are separable in the spatial frequency domain.

5. The darkfield digital holographic microscope of any of claims 3 to 4, configured such that the first and second reference radiation beams are arranged to each be incident at a respective different angle of incidence relative to an optical axis of the darkfield digital holographic microscope.

6. The dark field digital holographic microscope of any preceding claim, wherein the illumination device further comprises a coherence matching device operable to: adjustably delaying one of the first illumination beam and the first reference beam relative to the other of the first illumination beam and the first reference beam so as to maintain the beams of the first beam pair at least partially coherent; and adjustably delaying one of the second illumination beam and the second reference beam relative to the other of the second illumination beam and the second reference beam so as to maintain the beams of the second beam pair at least partially coherent.

7. The dark-field digital holographic microscope of claim 6, wherein the illuminating means comprises a time delay means configured to impose non-coherence between the first and second beam pairs by being operable to delay one of the first and second beam pairs relative to the other of the first and second beam pairs, wherein optionally the time delay means comprises an adjustable time delay means operable to adjustably delay one of the first and second beam pairs relative to the other of the first and second beam pairs to impose the non-coherence.

8. The dark-field digital holographic microscope of claim 7, wherein the illumination device includes a first branch operable to provide the first pair of beams and a second branch operable to provide the second pair of beams, and wherein:

the time delay means comprises at least a delay line operable to impose a delay on one of the first or second branches relative to the other of the first or second branches; and

the coherent matching device comprises: a first coherence matching device in the first branch, the first coherence matching device operable to adjustably delay at least one of the first reference beam and the first illumination beam; and a second coherence matching device in the second branch, the second coherence matching device being operable to delay at least one of the second reference beam and the second illumination beam in an adjustable manner.

9. The dark-field digital holographic microscope of claim 7, wherein the illumination device includes: a first branch operable to provide the first and second illumination beams; and a second branch operable to provide the first reference beam and the second reference beam; wherein:

implementing the coherent matching means and the time delay means via a joint optimization of at least: a first adjustable delay line operable to impose an adjustable delay on one of the first or second branches relative to the other of the first or second branches; a second adjustable delay line in the first branch, the second adjustable delay line operable to impose an adjustable delay on at least one of the first illumination beam and the second illumination beam; and a third adjustable delay line in the second branch, the third adjustable delay line operable to impose an adjustable delay on at least one of the first reference beam and the second reference beam.

10. The dark field digital holographic microscope of any preceding claim, wherein the first illuminating radiation beam is configured to illuminate the structure at a first angle of incidence; the second beam of illumination radiation is configured to illuminate the structure at a second angle of incidence different from the first angle of incidence.

11. The dark-field digital holographic microscope of any preceding claim, wherein the illumination device comprises a single radiation source, the illumination device being configured to generate the first and second beam pairs from the single radiation source.

12. The dark field digital holographic microscope of claim 11, wherein the single light source is configured to emit at least partially coherent radiation.

13. The dark-field digital holographic microscope of any preceding claim, wherein the illumination device is configured to generate the first and second reference beams, respectively, at a first power level, and to generate the first and second illumination beams, respectively, at a second power level, the second power level being greater than the first power level.

14. A method of determining a characteristic of interest of a target formed on a substrate by a lithographic process, the method comprising:

illuminating the target with a first beam of illumination radiation and capturing resulting first scattered radiation that has been scattered from the target;

illuminating the target with a second beam of illumination radiation and capturing resulting second scattered radiation that has been scattered from the target;

imposing spatial and/or temporal incoherence between a first beam pair comprising the first illumination beam and the reference beam and a second beam pair comprising the second illumination beam and the second reference beam such that:

the beams of the first beam pair are at least partially spatially coherent and at least partially temporally coherent,

the beams of the second beam pair are at least partially spatially coherent and at least partially temporally coherent, an

Any beam of the first beam pair is spatially incoherent and/or temporally incoherent with respect to any beam of the second beam pair; and

simultaneously generating a first interference pattern resulting from interference of the first scattered radiation with a first reference beam of radiation and a second interference pattern resulting from interference of the second scattered radiation with a second reference beam.

15. A metrology apparatus for determining a property of interest of a structure on a substrate, the metrology apparatus comprising a dark-field digital holographic microscope according to any one of claims 1 to 13.

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